Transistor Amplifiers Flashcards
do transistors amplify current or voltage?
Current! Although one can design a voltage amplifier out of transistors.
what happens when you put an A/C signal through a transistor?
not well - the bottom half of the waveform is cut off on the output.
what is ‘biasing’?
when an A/C signal is put through a transistor, the negative part of the waveform is cut off. To prevent this, one may add a DC bias voltage - literally a DC voltage used to raise the A/C signal so that the entire waveform can be put through the transistor without anything being cut off.
how is biasing achieved?
voltage divider, baby!
A/C voltage gain formula?
Rc / Re
What is an emitter bypass capacitor?
A capacitor in parallel with the emitter resistor (at output of transistor).
At high frequency (A/C signal), this acts as a short circuit and the emitter resistor is bypassed. For the DC bias, however, the emitter resistor is still required. This makes switching faster.
What’s a coupling capacitor, and what does it do?
a capacitor at the input of a transistor amplifier. It creates a high-pass filter (when freq. is high, Cc = short circuit, freq. = low, Cc = high impedence).
This means the gain changes with frequency! And there’s an amplifier - this is often expressed in dB (v(gain / decade)
What’s ‘bandwidth’ or ‘midband gain’?
If you look at the frequency response (Bode) plot of an amplfiier, there is an initial linear increase of 20dB/decade. This means different frequencies will be amplified at different rates! This is clearly not desirable. Therefore, an amplifier must operate within the ‘flat’ section of the bode plot (termed ‘bandwidth’).
