SERVICE

MODEL8559A

emitter is fixed by R8. The collector, therefore, becomes a constant-current sink for 20 rnA of current supplied by Q1 and 43. A decrease in the current supplied by 4 3 results in increased current through Q1, keeping the current through 4 2 constant. If an LC filtered bandwidth is selected, BW5F (filtered bandwidth control line 5 in block C) supplies current via CR1 and R13 (see Figure8-47); 4 3 is effectively removed from the circuit.

0 l O d B INPUT BUFFE R AMPLIFIE R

+15VF1

R q

CB

 

 

FROM BW5F

I N BLOCK C

FIGURE 8-47. DC BIASPATH DURING LC POLEOPERATION

To understand how 4 3 functions during crystal filtering modes, a new model is needed. (See Figure 8-48.) Resistor R7 has been omitted to simplify the model. The emitter load of 4 3 (RJ is the series combination of the internal resistance of Y1 (RJand a resistance determined by the bandwidth selected (see First Xtal Pole descrip- tion). The crystal's series resistance at resonance (RJ is constant at about 10 ohms. In the 30 kHz bandwidth, R23* is in series with k.Since R23* is very large by comparison, it represents the total load on 4 3 (RJ. When R23* is substituted into the gain equation for R,,a gain of 2.7 (8.6 dB) results. This is roughly equal to the gain without 4 3 in the circuit. In fact, the larger R,becomes, the closer the gains become.

Q lOdB INPUT

 

 

 

 

 

 

 

 

BUFFER

AMPLIFIE R

 

 

 

 

 

 

01,02

 

 

 

i t -R5tR6

 

 

 

 

 

OAIN-

 

 

R3

 

 

 

 

 

 

-

 

 

 

 

 

 

1-

 

 

 

 

 

 

 

 

 

A t

 

 

 

 

 

FOR

3 0 ~BW:

 

 

R5

 

R6

When: ~ t - 8 2 5 0n

I

 

T

-,

 

 

 

 

 

 

R3

FEEDBACK

 

 

 

 

 

 

 

 

 

I

 

 

 

 

185.7

 

 

 

 

 

 

 

110

 

- 2 . 8 9 OAIN 2 . 7 0 = B.6dB

 

 

 

 

Then: OAIN-

 

23 . 7

 

4'

 

 

0

1-

-

 

 

.

 

1

- 1

1

 

82.50

 

 

 

 

 

 

 

 

 

 

 

 

FOR

i K BW:

a

 

 

 

 

When:

Rt-70

 

 

 

 

Then:

OAIN- 2.8'- = 12dB

 

 

 

 

 

 

 

 

 

.€ia-'.O

FIGURE8-48. 10 dB INPUT BUFFER AMPLIFIER DURING CRYSTALFILTER OPERATION