Bradley 233 - Output Section

4.1.10 Transistor Driver
TR70 and D72 form a differential pair, differing from a transistor pair in that the source,
pin 9 and 11 has to provide the tail current through R72.
The current through R76 raises the output pulse so that its baseline is at earth potential.
4. 2 OUTPUT SECTION (Printed Circuit prefixes 200 and 300 Fig. 7)
4. 2.1 Amplitude
The output stages provide an essentially negative amplitude current pulse. TR35 is always
in operation, although the current it switches depends on the voltage across R35, set by
emitter follower TR40 and the amplitude vernier RV43. TR30, TR25 and TR20 come on in
succession as the amplitude switch is set from 2. 5V to 5, 7.5 and at lOV when all three are
in operation. A positive d. c. current equal to the maximum amplitude current (200mA) is
available from TR70 to provide positive baseline offset.
At full amplitude on positive pulse the situation is the same as on negative pulse with full
offset, but as the amplitude switch setting is reduced,TR20, TR25 and TR30 are held on to
maintain the baseline of the pulse at the same level. Similarly when the current through TR35
is reduced, which would cause the lower level of the output to rise, an equal increase in the
d. c. current through TR30 pulls down the pulse, again holding the baseline constant.
4.2.2 Input Stage
IClOa selects and combines the input signals by a positive OR function. IClOb provides
normal and complementary signals, R12 helping to prevent reflections in the track. TRIO
in combination with R15, R33 and R34 translates the logic levels down to those of IC20 and
IC30 which are based on the -12V supply. The in-phase signal through C14 provides a lower
source impedance for fast edges. Without this the output transition times would be several
times longer.
4.2.3 Offset Current
The sum of the voltages across R70 and R82 is always equal to the base-emitter drop of TR80.
Thus changes in TR7 base-emitter voltage and current^gate' due to self heating have no effect
on the offset current.
As the voltage on the wiper of the offset potentiometer is reduced from near +15V to earth,
the voltage at the emitter of TR70 increases. The feedback maintains TR80 base voltage
constant so that R81 and R82 work as a 'lever' with a pivot at TR80 base voltage. R86 is a
temperature dependent resistor designed to compensate for drifts in the positive pulse zero.
4. 2.4 Choke Network
If TR70 were directly connected to the output, its capacitance would slow the output edges
considerably. L63 converts this capacitance into a high impedance tuned circuit, that is one
in which the oscillatory current is small compared with the voltage across the L or C. R63
and R64 provide critical damping of oscillator currents and are arranged so that the circuit
presents a purely resistive impedance to the output. L61 blocks high frequency resonance
in L63.
4. 2. 5 Distributed Output
The collectors of the output transistors are separated by small printed circuit inductors.
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