AF Amplifier 1A12A6 Circuit Functioning

TM 11-5820-695-35

output impedance at 100 ohms. When the signal at the

(2) The input circuit of A2 is a sharply tuned,

base of Q2 goes positive, current through Q2 increases

crystal controlled bandpass filter comprising crystal Y1,

and the emitter voltage rises. Q2 functions essentially

capacitors C9, C14 and C17, inductor L5 and resistors

as a high impedance emitter follower but the current

R36 and R37. Capacitor C9 is used to peak tune the

increase also causes a drop in voltage at the junction of

circuit to the crystal frequency. Crystal Y1 and capacitor

collector load resistors R8 and R9. The signal voltage

C9 also block the dc at the input. The bandpass filter

tapped off at this point increases the forward bias of Q3.

has single-ended input and balanced output. Capacitor

Q3 is connected as a common emitter amplifier with R7

C17 is used to balance the circuit so that equal voltages

as collector load resistor but it functions as a current

are applied at pins 2 and 3 of A2. As the signals at pins

switch and provides a much greater increase in current

2 and 3 alternately go positive with respect to the dc

than the increase of current in Q2. Isolating resistor Rll

bias, A2 produces positive pulses at the output (pin 7).

connects the collector of Q3 to voltage check point E1.

(3)

The positive pulses at the output of

The level detector is comprised of C6, CR1,

A2 are rectified and filtered by CR11 and C20. This

positive voltage, applied through resistor R43 to the

CR2, C7, R12, and R13. Diodes CR1 and CR2 provide

base, forward biases transistor Q12. The output signal

voltage doubling of the rectified cable received

of Q12 is then Ov (logic 0); this signal, inverted by A6C,

orderwire signal CROW. The detected voltage is used

is applied to NOR gate A7A (para 2-5g). The time

for system monitoring.

constant of C20-R43 is long relative to the signal

frequency and it bridges over short gaps in the signal

The transmit orderwire signal COWIN is

such as many consecutive logic zeroes. As long as

developed across R14 and R15, and amplified by Q4.

cable traffic is active, the output of Q12 remains logic O

The gain of this amplifier is determined by the radio of

(normal).

When cable traffic fails, capacitor C20

R16 to R17. The low frequency cutoff is determined by

discharges through R43 and Q12, then Q12 returns to

C8, R17, and R18. Dc bias is determined by R15, R17,

cut-off. When Q12 turns off capacitor C21 charges from

and R18. The amplified output signal is coupled to

a positive voltage source in inverter A6C and A6C then

power amplifier Q5 and Q6 through lowpass filter

has a logic 1 input (alarm).

network C14, R18, and C13. This network has a cut-off

frequency of approximately 2kHz. Stages Q5 and Q6

Voltage Filtering Components.

Voltage

are connected as a power amplifier as described above

Filtering components L1, L2 and L3, C3, C4, C11, C12,

in a above. The output signal is coupled through

C15, and C16 limit the coupling of switching transients

resistor R23 and capacitor C12 to the load. Isolating

and other stray signals to the power supply circuits.

resistor R24 connects the collector of Q6 to voltage

check point E8.

2-45. AF Amplifier 1A12A6 Circuit Functioning (fig.

The level detector network C13, CR3, CR4,

5-34)

C14, R25 and R26 functions the same as the level

detector network for the receive amplifier discussed in b

The complete functional description of af amplifier

above.

1A12A6 is provided in paragraph 2-5e. Electrical

operation of discret piece parts is provided below.

Voltage filtering components L1 through L4,

and C15 through C22 limit the coupling of switching

The incoming cable extracted orderwire

transients and other stray signals to the power supply

signal CCEXOW is developed across R1, and is

circuits.

amplified by Q1. Dc bias is provided by R1, R2, R4,

and R5. The gain of the amplifier is determined by the

2-46.

Radio Amplifier-Detector 1A12A7 Circuit

ratio of R3 to R4 and the low frequency cutoff is

Functioning (fig. 5-35)

determined by C1, R4 and RS. The amplified output

signal is delivered to the power amplifier Q2, Q3,

The complete functional description of radio amplifier-

through resistor R6 and capacitors C2 and C3 which

detector 1A12A7 is provided in paragraph 2-f. Electrical

form an audio low pass filter.

The filter cut-off

operation of discrete piece parts is provided in a through

frequency is approximately 2 kHz. Transistors Q2 and

j below.

Q3 are a power amplifier which provide current gain but

Agc Slicer and Buffer Amplfier. The agc

no voltage gain; the circuit operates essentially as an

emitter follower.

Resistor R10 and capacitor C5

slicer is a dual transistor (Q4) emitter follower which

provides output coupling to the load with R10 setting the

reduces the amplitude of the reshaped

Change 1 2-84.1