Tektronix 175, Manual

Page: 34 / 124

Circuit Description — Type 575
Fig. 3-8. The Step Amplifier furnishes either current or voltage
steps to the input of the transistor under test.
B
Fig. 3-9. The emitter-follower (a) operates the same as the cath­
ode-follower (bl.
Fig. 3-10. By switching the output connections, the emitter-fol ­
lower of Fig. 3-9 (a) becomes a collector-loaded amplifier.
switch is open (no current through the collector), the no-load
output voltage must be — 15 volts.
The circuits of Fig. 3-9 (a) and Fig. 3-10 have maximum ­
current limitations which are different. The circuit of Fig. 3-9
(a) can supply much more current in the negative direction,
(making the ungrounded end of the load resistance negative)
than it can supply in the positive direction (through the 100-
ohm resistor).
By the same method, it can be shown that the circuit of
Fig. 3-10 can supply much more current in the positive direc­
tion than in the negative direction.
Since the path of the higher current through the load in
both circuits was always through the upper battery, the upper
battery must be able to deliver more current than that which
is required of the lower one.
The drawing of Fig. 3-11 shows the electron-current flow
through the circuit components as the Step Generator drives
a load resistance in the negative direction. The lower battery
supplies only the current which flows through the 100-ohm
resistor. The upper battery must supply current to the load
as well.
Fig. 3-11. Electron flow through the transistor V253 circuit when
negative-going steps are required.
Figure 3-12 is a simplified diagram of the output circuit of
the Step Amplifier. Note that the load resistance across the
output circuit is always the current-sampling resistor in series
with either a 1-ohm resistor (voltage steps) or the input of the
transistor under test (current steps). The feedback paths go
directly to vacuum-tube grids and do not load the output
circuit.
The maximum current the Step Amplifier will deliver to
an external load is 2.4 amperes of either polarity (ma-per-
Fig. 3-12. Simplified diagram of the output circuit of the Step
Amplifier.
3-8