Transistor Amplifier's

Thread Starter


Joined Nov 7, 2012
Hey I was just wondering if anyone could help me with a couple of questions I have?

Explain in detail, the effects of a Bypass Capacitor on the AC and DC voltages of a transistor amplifier

Explain in detail, the effects of a Load on the AC and DC voltages of a transistor amplifier

Explain the “phase” relationship of the AC input and output voltages in a transistor amplifier


Joined Feb 3, 2013
I'm on my tablet right now so typing isn't the greatest, so these answers may not be in as much detail as you would like.

The bypass capacitor is used to increase the gain, while keeping the beta value as stable as possible. The higher the emitter resistors are, the more beta stability you get. But... With a larger value of this resistor the gain is lower, so we put two in and short the larger one (since capacitors are an AC short). Like I said about the tablet, I will not show you the formula, but I'm sure you have it somewhere.

I'm not 100% sure about this one because I am an instrumentation guy and not an electrical engineer, but this is my best guess. If input and output capacitors are used, a load has zero effect on DC voltages, because capacitors are a DC blocker. It has a very minimal effect on AC voltage as well, as it can be seen as in parallel with the transistor.

Assuming you're using a common-emitter amplifier (since they are the most widely used), the output voltage will be 180 degrees out of phase with the input voltage. This is because when the input voltage goes up, base current increases, which increases collector current, which increases collector resistor voltage. Since the output is in parallel with the collector part and the ground, using KVL we find that this voltage must decrease.


Joined May 28, 2009
In my opinion, the bypass or DC blocking / bypass capacitor in an bipolar transistor amplifier will block DC voltage while allowing an AC sine wave (audio) signal to pass. The capacitor should not be polarized. The purpose of the DC blocking capacitor is to allow a specific bias voltage to be fed the base of the transistor, not using a typically higher voltage provided to a transistor in a previous stage.

The phase of the AC input to the base of an NPN bipolar transistor will be inverted at the collector but will not be inverted at the emitter.

It would help to clarify if you provided us with a schematic to see what we are trying blindly to explain.