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Figure 21 RF AMPLIFIER, MIXER, AND OSCILLATOR

STAGES FOR TRANSISTORIZED HIGH FREQUENCY RECEIVER. THE RCA 2N247 DRIFT TRANSISTOR IS CAPABLE OF EFFICIENT OPERATION UP TO 23 Mc.

о TO i .F. О AMPLIFIER


Transistor Sufficient coupling of the proper Oscillators phase between input and output circuits of the transistor will permit oscillation up to and slightly above the alpha cutoff frequency. Various forms of transistor oscillators are shown in figure 22. A simple grounded emitter Hartley oscillator having positive feedback between the base and the collector (22A) is compared to a grounded base Hartley oscillator (22B). In each case the resonant tank circuit is common to the input and output circuits of the transistor. Self-bias of the transistor is employed in both these circuits A more sophisticated oscillator employing a 2N247 transistor and utilizing a voltage divider-type bias system (figure 22C) is capable of operation up to 50 Mc. or so. The tuned circuit is placed in the collector, with a small emitter-collector capacitor providing feedback to the emitter electrode.

A P-N-P and an N-P-N transistor may be combined to form a complementary Hartley oscillator of high stability (figure 23)- The collector of the P-N-P transistor is directly

coupled to the base of the N-P-N transistor, and the emitter of the N-P-N transistor furnishes the correct phase reversal to sustain oscillation. Heavy feedback is maintained between the emitter of the P-N-P transistor and the collector of N-P-N transistor. The degree of feedback is controlled by Ri. The emitter resistor of the second transistor is placed at the

/ф\ /±\ .±-2N247 SiOK np INei** W1N81 J 2N

Figure 23 COMPLEMENTARY HARTLEY OSCILLATOR

P-N-P and N-P-N transistors form high stability oscillator. Feedback between P-N-P emitter and N-P-N collector is controlled by Rl. 1N81 diodes are used as amplitude lim-iters. Frequency of oscillation is determined by L. C-Cu


1-Э к

®

Figure 22

TYPICAL TRANSISTOR OSCILLATOR CIRCUITS

A-Grounded Emitter Hartley В-Grounded Base Hartley

C-2N247 Oscillator Suitable for SO Mc. operation.


;N247 Pnp

о

39 К

м-

--M-t-

--ni -I- r

;,oi :i;.oi

- -9V.

©



2N33

POINT-contact transistor

point-contact transistor


Figure 24 NEGATIVE RESISTANCE OF POINT-CONTACT TRANSISTOR PERMITS HIGH FREQUENCY OSCILLATION (50 Mc) WITHOUT WITHOUT NECESSITY OF EXTERNAL FEEDBACK PATH.

center of the oscillator coil to eliminate loading of the tuned circuit.

Two germanium diodes are employed as amplitude limiters, further stabilizing amplifier operation. Because of the low circuit impedances, it is permissible to use extremely high-C in the oscillator tank circuit, effectively limiting oscillator temperature stability to variations in the tank inductance.

The point-contact transistor exhibits negative input and output resistances over part of its operaing range, due to its unique ability to multiply the input current. This characteristic affords the use of oscillator circuitry having no external feedback paths (figure 24). A high impedance resonant circuit in the base lead produces circuit instability and oscillation at the resonant frequency of the L-C circuit. Positive emitter bias is used to insure thermal circuit stability.


с 1 CHARUINu PERIOD

*1 E

Ci Ri %R3


®

Figure 25 RELAXATION OSCILLATOR USING POINT-CONTACT OR SURFACE BARRIER TRANSISTORS.

Relaxation Transistors have almost unlimit-Oscillatars ed use in relaxation and R-C oscillator service. The negative resistance characteristic of the point contact transistor make it well suited to such application. Surface barrier transistors are also widely used in this service, as they have the highest alpha cutoff frequency among the group of alpha-less-than-unity transistors. Relaxation oscillators used for high speed counting require transistors capable of operation at repetition rates of 5 Mc. to 10 Mc.

A simple emitter controlled relaxation oscillator is shown in figure 25, together with its operating characteristic. The emitter of the transistor is biased to cutoff at the start of the cycle (point 1). The charge on the emitter capacitor slowly leaks to ground through the emitter resistor, Ri. Discharge time is determined by the time constant of RiCi. When the emitter voltage drops sufficiently low to permit the transistor to reach the negative resistance region (point 2) the emitter and collector resistances drop to a low value, and the collector

LOCMI

SIGN


6.8 к


--- 10 к

®

Figure 26

TRANSISTORIZED BLOCKING OSCILLATOR (A) AND ECCLES-JORDAN

BI-STABLE MULTIVIBRATOR (B). High-alpha transistors must be employed in counting circuits to reduce effects of storage time caused by transit lag in transistor base.



2N136

2N135 T2


1IM64

2N 135

2N107


Cl- 123JUiJF, J.M/. MILLER Л г.пО Сг - 79JUJUF, PART OF Cl

Ll- LOOPSTICK COIL, J.ty. MILLER * гООЗ La-OSCILLATOR COIL, J . MILLER # ZOOZ Tl -455 KC, I.F, TRANSFORMER, J-W. MILLER*203I Tz -45S KC. I.F. TRANSFORMER, J.W, MILLER * гОЗг

Figure 28

SCHEMATIC, TRANSISTORIZED BROADCAST BAND (500-1600 KC.) SUPERHETERODYNE RECEIVER.

LOOPSTICK COIL

1 N34A

г к

PHONES

-t-X

Figure 27 WRIST RADIO CAN BE MADE WITH LOOPSTICK, DIODE, AND INEXPENSIVE CK-722 TRANSISTOR. A TWENTY FOOT ANTENNA WIRE WILL PROVIDE GOOD RECEPTION IN STRONG SIGNAL AREAS.

current is limited only by the collector resistor, R;. The collector current is abruptly reduced by the charging action of the emitter capacitor Cl (point 3), bringing the circuit back to the original operating point. The spike of collector current is produced during the charging period of Cl. The duration of the pulse and the pulse repetition frequency (p.r.f.) are controlled by the values of Ci, Ri, R, and Ri.

Transistors may also be used as blocking oscillators (figure 26A). The oscillator may be synchronized by coupling the locking signal to the base circuit of the transistor. An oscillator of this type may be used to drive a flip-flop circuit as a counter. An Eccles-Jordan bi-stable flip-flop circuit employing surface-barrier transistors may be driven between off and on positions by an exciting pulse as shown in figure 2бБ. The first pulse drives the on transistor into saturation. This transistor remains in a highly conductive state until the second exciting pulse arrives. The transistor does not immediately return to the cut-off

state, since a time lapse occurs before the output waveform starts to decrease. This storage time is caused by the transit lag of the minority carriers in the base of the transistor. Proper circuit design and the use of high-alpha transistors can reduce the effects of storage time to a minimum. Driving pulses may be coupled to the multivibrator through steering diodes as shown in the illustration.

5-6 Transistor Circuits

With the introduction of the dollar transistor, many interesting and unusual experiments and circuits may be built up by the beginner in the transistor field. One of the most interesting is the wrist watch receiver, illustrated in figure 27. A diode and a transistor amplifier form a miniature broadcast receiver, which may be built in a small box and carried on the person. A single 1.5-volt penlite cell provides power for the transistor, and a short length of antenna wire will suffice in the vicinity of a local broadcasting station.

A transistorized superhetrodyne for broadcast reception is shown in figure 28. No antenna is required, as a ferrite loop-stick is used for the r-f input circuit of the 2NI36 mixer transistor. A miniature magnetic hearing aid type earphone may be employed with this receiver.

A simple phonograph amplifier designed for use with a high impedance crystal pickup is shown in figure 29- Two stages of amplification using 2N109 transistors are used to drive two 2N109 transistors in a class В configuration. Approximately 200 milliwatts of



2 го к -W-1

2N109 0 15 M


2N109

P 2N109 Zp=z5K

f: , ог7м Zs = 3K, c.T.


----ш--

lOOJUF IlOOJUF

-1 г V.

4-50 MA.

Figure 29

HIGH GAIN, LOW DISTORTION AUDIO AMPLIFIER, SUITABLE FOR USE WITH A CRYSTAL PICKUP. POWER OUTPUT IS 250 MILLIWATTS.

power may be obtained with a battery supply esting transistor projects will be shown in later of 12 volts. Peak current drain under maxi- chapters of this Handbook, mum signal conditions is 40 ma. Other inter-



E & E TECHNI-SHEET

TABLE OF WROUGHT STEEL STANDARD PIPE (BLACk OR GALVANIZED) RANDOM LENGTHS, 20-21 FEET

NOMINAL SIZE

INCHES

O.D.

INCHES

INCHES

POUNDS PER FOOT

1/8

.405

.269

.244

1/4

.540

.364

.424

3/8

.675

.493

.567

.840

.622

.850

3/4

1.050

.824

1.130

1 .31 5

1 .049

1 .678

1 1 /4

1 .660

1 .380

2.272

1 1/2

1 .900

1.610

2 .71 7

2.375

2 .067

3 .652

2 1 / 2

2.875

2.469

5 .793

3.500

3 .068

7.575

3 1/2

4.000

3.548

9 .109

4.500

4.026

10.790

TABLE OF ELECTRICAL METALLIC TUBING (EMT) GALVANIZED, 10 FOOT LENGTHS

NOMINAL SIZE

1NCHES

O.D.

INCHES

I.D. INCHES

POUNDS PER FOOT

3/8

.577

.493

.250

1/2

.706

.622

.321

3/4

.922

.824

.488

1.163

1 .049

.71 1

1 1 /4

1.508

1.308

1.000

1 1 /2

1 .738

1 .6 1 0

1.180

2.195

2.067

1 .500

TABLE OF ALUMINUM ROD, 12 FOOT LENGTHS

DIAMETER

WT. it/FT.

1 /8

.01 5

3/16

.033

1/4

.059



CHAPTER SIX

Vacuum Tube Amplifiers

Vacuum Tube Parameters

Electrode Potentials

The ability of the control grid of a vacuum tube to control large amounts of plate power with a small amount of grid energy allows the vacuum tube to be used as an amplifier. It is this abiliry of vacuum tubes to amplify an extremely small amount of energy up to almost any level without change in anything except amplitude which makes the vacuum tube such an extremely valuable adjunct to modern electronics and communication.

Symbols for As an assistance in simplify-Vocuom-Tube ing and shortening expressions Parameters involving vacuum-tube parameters, the following symbols will be used throughout this book:

Tube Constants

fi - amplification factor Rp - plate resistance Gfa -transconductance

- grid-screen mu factor

- conversion transconductance(mixer tube)

Interelectrode Capacitances

Cgk - grid-cathode capacitance Cgp - grid-plate capacitance Cpj -plate-cathode capacitance Cjn-input capacitance (tetrode or pentode)

supply voltage (a positive (a negative

Ebb-d-c plate

quantity) Ecc - d-c grid supply voltage

quantity)

Egp,-peak grid excitation voltage total

peak-to-peak grid swing) Epm -peak plate voltage {У2 total peak-to-peak

plate swing) ep - instantaneous plate potential eg -instantaneous grid potential pmin -minimum instantaneous plate voltage

maximum positive instantaneous grid

voltage Ep - static plate voltage Eg - static grid voltage eo - cutoff bias

Electrode Currents

lb - average plate current

If. - average grid current

Ipn, -peak fundamental plate current

ipmax -maximum instantaneous plate current

igmax-maximum instantaneous grid current

Ip - static plate current

Ig - static grid current

Other Symbols

Pj - plate power input Po - plate power оифиг p-plate dissipation

(in-input capacitance (.tetroae or pentoaej i-p-piate oissipauon

Cout -output capacitance (tetrode or pentode) Pj - grid driving power (grid plus bias losses)



Classes of Amplifiers 107

Cgp~

==--цЛ.

;CouT

TRIODE

PENTODE OR TETRODE

Figure 1

STATIC INTERELECTRODE CAPACITANCES WITHIN A TRIODE, PENTODE, OR TETRODE

Pg -grid dissipation

Np -plate efficiency (expressed as a decimal) вр - one-half angle of plate cmrent flow $g - one-half angle of grid current flow Rl -load resistance Zl - load impedance

Vacuutn-Tube The relationships between cer-Constants tain of the electrode potentials and currents within a vacuum tube are reasonably constant under specified conditions of operation. These relationships are called vacuum-tube constants and are listed in the data published by the manufacturers of vacuum tubes. The defining equations for the basic vacuum-tube constants are given in Chapter Four.

interelectrode The values of interelectrode Capacitances and capacitance published in Miller Effect vacuum-tube tables are the

static values measured, in the case of triodes for example, as shown in figure 1. The static capacitances are simply as shown in the drawing, but when a tube is operating as amplifier there is another consideration known as Miller Effect which causes the dynamic input capacitance to be different from the static value. The output capacitance of an amplifier is essentially the same as the static value given in the published tube tables. The grid-to-plate capacitance is also the same as the published static value, but since the Cgp acts as a small capacitance coupling energy back from the plate circuit to the grid circuit, the dynamic input capacitance is equal to the static value plus an amount (frequently much greater in the case of a triode) determined by the gain of the stage, the plate load impedance, and the Cgp feedback capacitance. The total value for an audio amplifier stage can be expressed in the following equation:

{dynaiO (static) (д^ where Cgt the grid-to-cathode capacitance.

Cgp is the grid-to-plate capacitance, and A is the stage gain. This expression assumes that the vacuum tube is operating into a resistive load such as would be the case with an audio stage working into a resistance plate load in the middle audio range.

The more complete expression for the input admittance (vector sum of capacitance and resistance) of an amplifier operating into any type of plate load is as follows:

Input capacitance = Cgt -i- (1 + A cos в) Cgp

Input resistance =--- \

A sin 9

Where: Cgt = grid-to-cathode capacitance Cgp = grid-to-plate capacitance A = voltage amplification of the tube alone

в = phase angle of the plate load impedance, positive for inductive loads, negative for capacitive

It can be seen from the above that if the plate load impedance of the stage is capacitive or inductive, there will be a resistive component in the input admittance of the stage. The resistive component of the input admittance will be positive (tending to load the circuit feeding the grid) if the load impedance of the plate is capacitive, or it will be negative (tending to make the stage oscillate) if the load impedance of the plate is inductive.

Neutrolizattan Neutralization of the effects of Interelectrode of interelectrode capacitance Capacitance is employed most frequently

in the case of radio frequency power amplifiers. Before the introduction of the tetrode and pentode tube, triodes were employed as neutralized Class A amplifiers in receivers. This practice has been largely superseded in the present state of the art through the use of tetrode and pentode tubes in which the Cgn or feedback capacitance has been reduced to such a low value that neutralization of its effects is not necessary to prevent oscillation and instability.

6-2 Classes and Types of Vacuuni>Tube Amplifiers

Vacuum-tube amplifiers are grouped into various classes and sub-classes according to the type of work they are intended to perform. The difference between the various classes is determined primarily by the value of average grid bias employed and the maximum value of



the exciting signal to be impressed upon the grid.

Class A A Class A amplifier is an amplifier Amplifier biased and supplied with excitation of such amplitude that plate current flows continuously (360° of the exciting voltage waveshape) and grid current does not flow at any time. Such an amplifier is normally operated in the center of the grid-voltage plate-current transfer characteristic and gives an output waveshape which is a substantial replica of the input waveshape.

Class Aj This is another term applied to the Amplifier Class A amplifier in which grid current does not flow over any portion of the input wave cycle.

Class A2 This is a Class A amplifier oper-Amplifier ated under such conditions that the grid is driven positive over a portion of the input voltage cycle, but plate current still flows over the entire cycle.

Class ABj This is an amplifier operated under Amplifier such conditions of grid bias and exciting voltage that plate current flows for more than one-half the input voltage cycle but for less than the complete cycle. In other words the operating angle of plate cur-tent flow is appreciably greater than 180° but less than 360°. The suffix 1 indicates that grid current does not flow over any porrion of the input cycle.

Class AB2 A Class AB2 amplifier is operated Amplifier under essentially the same conditions of grid bias as the Class ABj amplifier mentioned above, but the exciting voltage is of such amplitude that grid current flows over an appreciable portion of the input wave cycle.

С

®


--IP

©

Figure 2 TYPES OF BIAS SYSTEMS

A - Grid bias

В - Cathode bias

С - Grid leak bias

less than one-half the time. Actually, the conventional operaring conditions for a Class С amplifier are such thar plate current flows for 120° to 150° of the exciting voltage waveshape.

Types of There are three general types of Amplifiers amplifier circuits in use. These types are classified on the basis of the return for the input and output circuits. Conventional amplifiers are called cathode return amplifiers since the cathode is effectively grounded and acts as the common return for both the input and output circuits. The second type is known as a plate return amplifier or cathode follower since the plate circuit is effectively at ground for the input and output signal voltages and the output voltage or power is taken between cathode and plate. The third type is called a grid-return or grounded-grid amplifier since the grid is effectively at ground potential for input and ouфut signals and output is taken between grid and plate.

Biosing Methods

Class В A Class В amplifier is biased sub-Ampfifier stantially to cutoff of plate current (without exciting voltage) so that plate current flows essentially over one-half the input voltage cycle. The operating angle of plate current flow is essentially 180° The Class В amplifier is almosr always excited to such an extent that grid current flows.

Gloss С A Class С amplifier is biased to a Amplifier value greater than the value required for plate current cutoff and is excited with a signal of such amplitude that grid current flows over an appreciable period of the input voltage waveshape. The angle of plate current flow in a Class С amplifier is appreciably less than 180° or in other words, plate current flows appreciably

The difference of potential between grid and cathode is called the grid bias of a vacuum tube. There are three general methods of providing this bias voltage. In each of these methods the purpose is to establish the grid at a potential with respect to the cathode which will place the tube in the desired operating condition as determined by its characteristics.

Grid bias may be obtained from a source of voltage especially provided for this purpose, as a battery or other d-c power supply. This method is illustrated in figure 2A, and is known as fixed bias.

A second biasing method is illustrated in figure 2B which utilizes a cathode resistor across which an IR drop is developed as a result of plate current flowing through it. The



HANDBOOK

Amplifier Distortion 109

cathode of the tube is held at a positive potential with respect to ground by the amount of the IR drop because the grid is at ground potential. Since the biasing voltage depends upon the flow of plate current the tube cannot be held in a cutoff condition by means of the cathode bias voltage developed across rhe cathode resistor. The value of this resistor is determined by the bias required and the plate current which flows at this value of bias, as found from the tube characteristic curves. A capacitor is shunted across the bias resistor to provide a low impedance path to ground for the a-c component of the plate current which results from an a-c input signal on the grid.

The third method of providing a biasing voltage is shown in figure 2C, and is called grid-leak bias. During the portion of the input cycle which causes the grid to be positive with respect to the cathode, grid current flows from cathode to grid, charging capacitor С„. When the grid draws current, the grid-to-cathode resistance of the tube drops from an infinite value to a very low value, on the order of 1,000 ohms or so, making the charging time constant of the capacitor very short. This enables Cg to charge up to essentially the full value of the positive input voltage and results in the grid (which is connected to the low potential plate of the capacitor) being held essentially at ground potential. During the negative swing of the input signal no grid current flows and the discharge path of Cg is through the grid resistance which has a value of 500,000 ohms or so. The discharge time constant for Cg is, therefore, very long in comparison to the period of the input signal and only a small part of the charge on Cg is lost. Thus, the bias voltage developed by the discharge of Cg is substantially constant and the grid is not permitted to follow the positive portions of the input signal.

Distortion in Amplifiers

There are three main types of distortion that may occur in amplifiers: frequency distortion, phase distortion and amplitude distortion.

Frequency Frequency distortion may occur Distortion when some frequency components of a signal ate amplified more than others. Frequency distortion occurs at low frequencies if coupling capacitors between stages are too small, or may occur at high frequencies as a result of the shunting effects of the distributed capacities in the circuit.


OUTPUT SIGNAL

Phase Distortion

In figure 3 sisting of third

an input signal con-a fundamental and a harmonic is passed through

Figure 3

lllustrafion of the effect of phase distortion on input wave containing a third harmonic signal

a two stage amplifier. Although the amplitudes of both components are amplified by identical ratios, the output waveshape is considerably different from the input signal because the phase of the third harmonic signal has been shifted with respect to the fundamental signal. This phase shift is known as phase distortion, and is caused principally by the coupling circuits between the stages of the amplifier. Most coupling circuits shift the phase of a sine wave, but this has no effect on the shape of the output wave. However, when a complex wave is passed through the same coupling circuit, each component frequency of the waveshape may be shifted in phase by a different amount so that the ouфut wave is not a faithful reproduction of the input waveshape.

Amplitude If a signal is passed through a vac-Distortion uum tube that is operating on any non-linear part of its characteristic, amplitude distortion will occur. In such a region, a change in grid voltage does not result in a change in plate current which is directly proportional to the change in grid voltage. For example, if an amplifier is excited with a signal that overdrives the tubes, the resultant signal is distorted in amplitude, since the tubes operate over a non-linear portion of their characteristic.

6-5 Resistance-

Capacitance Coupled Audio-Frequency Amplifiers

Present practice in the design of audio-frequency voltage amplifiers is almost exclusively to use resistance-capacitance coupling between the low-level stages. Both triodes and




Figure 4

STANDARD CIRCUIT FOR RESISTANCE-CAPACITANCE COUPLED TRIODE AMPLIFIER STAGE

pentodes are used; triode amplifier stages will be discussed first.

R-C Coupled Figure 4 illustrates the stand-Triode Stages ard circuit for a resistance-capacitance coupled amplifier stage utilizing a triode tube with cathode bias. In conventional audio-frequency amplifier design such stages are used at medium voltage

levels (from 0.01 to 5 volts peak on the grid of the tube) and use medium-/: triodes such as the 6J5 or high-;/ triodes such as the 6SF5 or 6SL7-GT. Normal voltage gain for a single stage of this type is from 10 to 70, depending upon the tube chosen and its operating conditions. Triode tubes are normally used in the last voltage amplifier stage of an R-C amplifier since their harmonic distortion with large output voltage (25 to 75 volts) is less than with a pentode tube.

Voltage Gain The voltage gain per stage of per Stage a resistance-capacitance coupled triode amplifier can be calculated with the aid of the equivalent circuits and expressions for the mid-frequency, high-frequency, and low-frequency range given in figure 5-

A triode R-C coupled amplifier stage is normally operated with values of cathode resistor and plate load resistor such that the actual voltage on the tube is approximately one-half the d-c plate supply voltage. To

E = -.


il Rl Rg

Rp {RL-bRc)+RLRG

MID FREQUENCY RANGE

Rp

:;Cpk :

;Rg ?

JU=-JUEgQ

:CcK

(dynamic, next stage)

HIGH FREQUENCY RANGE

A high freq.

A mio freq. 1+ (REQ/Xs)2

Req =

Rl Rl

Rg Rp

i

ZTTf (Cpk-(-Cgk (dynamic)

E=-JUEg (O,.


LOW FREQUENCY RANGE

A low FREQ. 1

A MID FREQ. t + (XC/R)2

1

R = Rg +

2ГТрСс

Rl Rp

Ru+ RP

Figure 5

Equivalent circuits and gain equations for a triode R-C coupled amplifier stage. In using these equations, be sore fo select the values of mu and Rp which ore proper for the static current and voltages with wbtch the tube will operate. These values may be obtained from curves published

in the RCA Tube Handbook RC-16.




1 ... 7 8 9 10 11 12 13 ... 80

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