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# KCC's quizzes AQQ235 about a bipolar common emitter amplifier - a kind proposal from our colleague Martin Walker

Apologize for our non-technical audience since this quiz is more for our FAEs...

A kind proposal from our colleague Martin Walker, ADI Product Marketing Engineer, UK:

Most of our electronic engineers have seen this sort of circuit in their first year study time using a bipolar transistor in its 3 famous basic configurations: common emitter, common collector and common base.

Here above is a common emitter configuration.

Conditions:

• Vcc = 6V
• Vout = 3V
• Tc = 25°C

Q1 is a BJT NPN with current gain β of several hundreds.

RB1 and RB2 are large compared to RC and RS

Questions :

1. Is there enough information to work out the gain of this circuit?
2. If so, what is the voltage gain of the circuit?
3. Is it a good amplifier?
4. If not, what would you do to mitigate for its limitations?

Again, many thanks Martin!

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[edited by: emassa at 2:19 PM (GMT -4) on 22 May 2023]
Parents
• Yes, but only for the start after power-up :-)

Interpreting Vout as output bias voltage, Ic=(Vcc-Vout)/Rc=3V/Rc

gm=Ic/Ut with Ut=kT/e=25.7mV

Small signal input resistance is rin=(beta*1/gm)||Rb1||Rb2

vu=rin/(rs+rin)*(-Rc*gm)

Is it a good amplifier: No, the bias point is not stable. The easiest way would be, to add an emitter resistor with a value 5 to 10 * 1/gm