HP 400E Operating And Service Manual Download Page 19

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Models 400E/400EL

4-14. The output from the collector of A2QI3 is

coupled through the Meter Bridge and fed back to the

emitter of A2Q9. A2C28 in the feedback circuit

adjusts the amount of feedback at the high end of the

frequency range, and A2R38 adjusts the feedback at

the  low  end.  This  calibrates  the amplifier  gain  at  both
ends  of  the  frequency  range.  A2R44,  45  and  72  are
switched  into  the  feedback  circuit  on  the 0.00 I  volt

range,  boosting the gain on that range. A2R44  adjusts
the  gain  on  the  I mV  range  with  a  400Hz  input.
A2R3 1 adjusts  the dc bias level of  the amplifier.

4·15. METER BRID GE.

4·16. Figure 4·2 shows a partial schematic of the

Meter Bridge. The meter bridge rectifies the ac

amplifier output and supplies the dc current to drive

the meter. In order to use part of the meter bridge
output as the rear terminal dc output, the meter has

to be referenced to ground. Transistor A2QI4

references the meter to ground.

4-17. During the positive half cycle, A2CR 15

conducts. Part of the current (solid line) goes through

A2C34 into the feedback path, and part of the

current  goes  through  A2 R53  and  the  meter  to
ground.  The  current  through  A2R53  turns  on
A2Q14,  and  A2QI4  draws  current  from  the  positive
supply.  The  current  from  A2Ql4  goes  through
A2C36  into  the  feedback  path.  The  current  through
A2QI4  and  A2C36  is  equal  to  the  current  drawn

through the meter, so the current out of the bridge is
equal to the current into the bridge.

4·18. During the negative half cycle, A2CRI 6
conducts and draws current from the feedback path

(dotted line). Part of the current goes through A2C36

Section IV

and  A2CR 16  into  the  amplifier,  and  part  goes
through  A2R53  and  the  meter to  ground.  The  current
through  A2R53  turns  on  A2Q14,  and  the  current
from  A2Q 14  goes  through  A2R54  and  A2CR 16  to
the  amplifier.  Again  the  current  through  the  meter
equals  the  current  through  A2R54,  and  the  current
into the bridge equals the current out.

4-19.  Transistor  A2Q 14  replaces  current  drawn  by
the  meter,  so  the  meter  bridge  is  kept  floating  while
the  meter  is  referenced  to  ground.  The  dc  output,

taken across A2R65 and R2, is also referenced to
ground.

4·2D. AC 0 UTPUT CIRCUIT.

4-21.  The  ac  output  circuit  isolates  the  meter b ridge
and  amplifier  from  the  ac  output  load.  It  consists  of
two emitter followers (A2QI  5 and Q 16) connected in
cascade.  A2R59 in  the base  circuit  of A2Q 15  zeroes
the output  dc level  at  the ac  output.

4-22. POWER SUPPL

4·23. The power supply produces regulated +26 volts

and -26 volts. Breakdown diode A2CR 7 establishes a

reference voltage of 6.98 volts. Part of the power
supply output is applied to the base of A2Q2, and

A2Q2 senses the difference between the supply

output and the reference. If the output voltage

changes, the emitter to base voltage of A2Q2 will
change; and the output of A2Q2 will change the

current through A2Q

the regulator.

4-24. The negative regulator, A2Q3 and A2Q4, uses
the +26 volt output as a reference. Consequently, the

negative supply is dependent upon the positive

supply.