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Bias Supplies
The -12 VTV is supplied to A4A3 from A4A2. This voltage determines log diode bias
current and becomes more negative as temperature increases to compensate for the increased
resistance of the log diodes. The same supply is used to compensate the log diodes of the four
log stages in A4A2.
LINEAR Mode
In LINEAR Mode (See Figure 1) log diodes CR15 and CR16 are zero biased, and the limiting
action is removed from the stage. RT is large, and stage gain is nearly unity. Signal flow is
through emitter follower Qll and R48 to Q13.
L O G / L I N
L O G M O D E = +7V
L I N M O D E = - 1 2 V
Log Stages
All nine of the log amplifier stages in A4A2 and A4A3 are similar in operation, although
there are variations in certain stages. While all stages except the first three contain two log
diodes each, the first two stages contain six diodes each, and the third contains four diodes.
More diodes are used in Stages 1 through 3 because these stages have their logging action at
higher signal levels than the other log stages. Log diodes of Stages 1 and 2 are biased so that
the logging action occurs at signal levels of approximately 0 dBm to $10 dBm, whereas the
remaining stages log at lower signal levels. To provide proper log fidelity (log amplifier output
versus input), attenuation is required between the output of a higher level log stage and the
input of a lower level log stage. Resistive divider R20, R29, and R30 attenuates the output of
Stage 2, and a similar attenuator is used between Stages 3 and 4. Resistors in series with the
log diodes of Stage 1 through 3 are factory-selected to compensate for variations in log diode
resistance. Correct stage gain is required for proper log fidelity. Refer to the A4A3 schematic
note for the method of checking the gain of Stages 1 through 5 in log mode.
2 A4A3
3 1 6
R 4 9
C R 2 5
\ I
Figure 1. Log Amplifier Stage, Simplified Schematic
C R 2 6
R 5 3
- 1 2 V T V
T O L O G A M P L I F I E R
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