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AMPLIFIER AND HIGH SPEED DECADE ASSEMBLY 19-0605 Fig.4.8
5.30
Input signals in the range 10Hz - 80 MHz are coupled to the amplifier board via
pin 31 to the gate of the F.E.T. Q5. Q5 is biased to zero volts by the resistors
in the attenuator assembly and Q6 provides a constant current source.
5.31
Q5 is connected as a source follower and with the emitter follower Q7 provides a
high input impedance to low impedance conversion at near unity gain.
5.32
D4, Q3, D5 and Q4 form a Umiting circuit by clipping the positive and negative
excursions of the input signal.
5. 33
The output of Q7 is coupled via C1 and C2 to the feedback amplifier Q8 and Q9.
The output of Q9 is coupled via C9 and C1 0 to a second feedback amplifier Q11
and Q12. This arrangement of feedback amplifiers provides a well defined gain at the
collector of Q12.
5.34
Q13 is an emitter follower stage which provides a low impedance source for the
Schmitt trigger circuit Q14 and Q15. The emitter of Q13 provides a convenient
test point for the amplifier stages. R26 in the base circuit of Q13 provides adjustment of the
Schmitt trigger input bias to the base of Q14.
5.35
Q14 and Q15 shape the waveform into square pulses with fast rise time edges which
are coupled via the emitter follower Q16 to the differentiating network C29 and R38.
5.36
The negative differentiated pulse only is amplified by Q18, thus a narrow pulse
appears at the collector of Q18 for every cycle of the amplifier input signal.
5.37
The output of Q18 is applied, by the function logic, to either JC3b or the Control
Board Assembly via pin 14 of the p.c.b.
Check Signal
5.38
The 100kHz check signal is applied via pin 13 of the p.c.b. to pin 10 of IC1b.
With the front panel OPERATE/SELF-CHECK switch in the SELF-CHECK position
IC1b gate opens and applies the 100kHz self-check signal to the base of Q8.
Auxiliary -6.2V Supply
5.39
An auxiliary 6.2V negative supply is provided to supplement the battery or mains
pack supply to the F.E.
T.
Q5, Q13, Q14 and Q15. The 6.2V negative supply
is generated by Q17, Q19 and
Tl. A 100kHz output from IC3 of the Control Assembly
is.
applied to T1 via Q17 and Q19.
5.40
D9 and D11 on the secondary of T1 rectify the signal and provide a -6.2 volts via
the smoothing circuit to Q14, Q15, Q13 and Q6, the constant current regulator
for Q5.
5-6
9057/9059
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