[HomeBrew] Class B Small Signal Amplifier Question

[Ian Wilson]

Hi Eugene,

It would be helpful to know a bit more about your circuit. I assume there
are 4 resistors:
  - collector to Vcc, "Rc"
  - Vcc to base, "Rb1"
  - base to ground, "Rb2"
  - emitter to ground, "Re"

The DC levels would be useful also, including the supply voltage Vcc.

With 1.5V on the base, the emitter voltage will be about (1.5-0.7) = 0.8V.
For Class A, Re should be chosen to set the collector current at something
suitable. For 10mA, this would require an 82 ohm resistor. 100 ohm would
be close enough for this experiment.

The base bias resistors should be chosen to pass about 10x the base current
of the transistor. Assume a beta of 150 and a collector current of 10mA. Then
base current will be about 70uA. Let's use 1mA for the base resistor
calculations.
Rb2 (base to ground): 1.5V with (1mA-70uA) flowing - approximately 1.6k. Rb1
(Vcc to base): (9-1.5)=7.5V with 1mA flowing - approximately 7.5k.

Also for Class A, the load resistor needs to place the collector voltage
somewhere about 1/2 (Vcc - Ve). If you are using a 9V supply, the collector
voltage will be about 4.1V. A load resistor of 410 ohm would be required. For
this experiment, 470 ohms would be close enough.

Is the emitter resistor bypassed? If not, the voltage gain of the
circuit will be
approximately Rc/Re = 470/100 = 4.7. If it is bypassed, the gain will be
considerably higher (and difficult to calculate at 7MHz without knowledge of
the various capacitances).

At 7MHz with an unbypassed emitter resistor I would expect the gain to be
reasonably close to 4 when measured with a scope probe.

If your input is 900mV peak, you may already be close to clipping on one
of the output peaks. You can try reducing the input somewhat and see
whether the output changes in proportion.

Once this is all working we can move on to changing the bias arrangements
for other modes of operation.

Hope this helps,

73, ian K3IMW