Buffer Amplifier; Output Amplifier And Vernier Attenuators - HP 8082A Operating And Service Manual

4 - 6
4 - 3 0 Let us assume to start with
constant current sources (iS1, I%?, IS3 and 154) for
we are operating with transition times in the range
1-5ns.
If twe also assume that the constant current
4-31
source5 IT5 and I LE are set by the transition time con-
trols to produce lOmA and 20mA respectively, then
the previous stage, the level shifter, is input to the slope
generator as shown at a and b, then at time
tor Ql turns on and Q2 turns off. Thus the only current
source that can now supply constant current sink ISUM
is ITE (lOmA). Thus the deficiency of 2QmA is made up
by the intrinsic capacitance of Ql which discharges. This
provides the slope of @ from tl to 12 When the voltage
has dropped to the level at t2 {slightly lower than the
potential of voltage source V2), diode CR3 starts to con-
duct and prevents the voltage from falling any further.
4 - 3 2 At time t3# transistor CM turns off and Q2
turns on. The current source IT5 (lOmA)
drain into ISUM and so it starts to recharge the intrinsic
capacitance of Ql (slope of @ from t3 to t4). When the
voitage level of point @ reaches t4 (a slightly higher
potential than voltage source Vl), diode CR1 starts to
conduct and holds the voltage at this level.
4 - 3 3 The same action as described above controls
the voltage levels at point @ Thus it can be seen that
the transition times of the pulses at points
depend on the currents from the two sources (ITE and
of the front panel controls.
4 - 3 4 The above description of the action of the
Slope Generator can easily be applied to the actual
4 - 3 5 If transition times greater than 5ns are re
quired, external capacitors must be switched in to
ment the intrinsic capacitances of Ql and Q2. These capa-
citors are Cl5 to C22 in Service 8heet 2a. Current sources
current path as shown in Figure 4-6 in order
diodes CR5 and CR6 forward biassed.

BUFFER AMPLIFIER

The outputs of the Slope Generator (normal
4-37
and complement) are input to one of two Buffer
that the four
the
cannot now
fiers dependent on the setting of the transition time con-
trols (1 to 5ns or > 5ns). The Buffer Amphf ier for the
fast ranges is in the same IC as the Slope Generator (see
Service Sheet 2b). The Buffer Amplifier for the slower
ranges is on board A8 in an identical
Slope Generator part inhibited. The changeover is
accomplished by switching the current sources and
-25V supplies of the U5 end A8Ul. Refer to Q33,
34.42. 43 on Service Sheet 3a. As shown in Service
Sheet 2b, the circuits in the Buffer Amplifier provide
a lowto-high impedance converter, a linear amplifier
and a facility for normal/complement switching.
OUTPUT AMPLl Ff ER AND
VERNIER ATTENUATORS
4 - 3 9 The attenuators operate on the current-shar-
ing principle of a diff erentiai amplifier. As figure 4-7
shows, each attenuating element is a differential
fier connected so that the signal current flows into (or
out of) the commonecl emitters. The output current is
taken from one of the collectors.
i
' F I X E D
Figure 4-7. Attenuator Element
4 - 4 0
ing in each collector is proportional to the potential
difference between the two bases. Thus, by varying the
sistors the current iQUT can be controlled. However,
because of the dependence on the base-emitter voltage
of each transistor the attenuator has a logarithmic
characteristic.
4-41 There are two attenuating elements per
attenuator because of the two outputs (one for each
channel) of the output differential amplifier. An arrange
ment is st-rrwn in figure 4-8.
I