HP 8559A Operation And Service Manual page 415

MODEL 8559A
LOG AMPLIFIER ASSEMBLY A14, CIRCUIT DESCRIPTION
The Log Amplifier Assembly A14 includes seven amplifier stages, each capable of providing linear and loga-
rithmic amplification. A detector circuit following the amplifier stages detect.7 the amplified 21.4 MHz IF
signal, producing the vertical display signal. The offset circuit that follows the detector operates in Log mode to
offset the vertical display signal in 100 mV steps. This steps the display in four 10-dB increments of apparent
gain and adds the last 40 dB of displayed step gain to the gain (50 dB) already provided in the IF section.
The seven amplifier stages are similar in operation. Different stages are selected as linear or log amplifiers,
depending on the setting of the Amplitude Scale switch.
Log Mode of Operation.
signal level increases. Limiting starts with stage seven, since it sees the combined gains of the other stages, and
continues sequentially as the signal level increases. Stage one is the last stage to begin limiting the signal. The
total limiting process provides 70 dB of log display range. Each stage consists of an emitter follower voltage-
driver and a common-base amplifier in which the gain is signal-level dependent. Increases in signal level
decrease the gain.
A simplified schematic of a typical log stage (the second stage) is shown in Figure 8-64. In Log mode, the LOG/
LIN control line is high (about
all of the other stages. Diodes CRlO and CRll are Schottky diodes with a forward bias voltage of approxi-
mately 0.4V Emitter follower 413 is a voltage source that develops signal current flow through CRlO and
.
This signal-current drives 420, a common-base amplifier tuned to approximately 21.4 MHz. The gain of
this amplifier is set by the ratio of R52 to the total resistance, R,, between the emitters of 413 and 420
(primarily the resistance of CRlO and CR1 I). The formula for computing the gain in dB is:
(A) (C) (D) (E) (F) (G) (H)
In Log mode, the gain of the seven amplifier stages is sequentially limited as the
+
Gain (dB)
=
SECOND STAGE
SIGNAL PATH
FIGURE 8-64. LOG MODE OPERATION, SIMPLIFIED SCHEMATIC
+
I
CONTROL L I N E
SERVICE