## Monday, March 9, 2009

## Thursday, March 5, 2009

### Frequency Response of BJT Amplifiers

# Low frequency response

Where R_{i} is the source resistance, R_{L} is the load resistance, R_{in} is the input resistance for your amplifier and R_{o} is the output resistance for your amplifier. These last two are calculated based on the type of amplifier you are working with (See the handout on small signal amplifier calculations).

Once you have calculated the frequencies due to C_{1} and C_{2}, the cutoff is determined by the following rules:

1)If the two frequencies are more than a decade apart then f_{low2} in Figure 1 (the 3db point of the amp) is simply the higher of the two values.

2)If the two frequencies are closer than one decade, then the actual cutoff frequency of the amp is somewhat larger than either of the two calculated frequencies.

3)If the amplifier has a bypass capacitor, then it can also influence the cutoff frequency. Typical, emitter bypass capacitors are chosen large enough so that their effects are negligible.

# High Frequency Response

As previously stated, the high frequency response of a discrete transistor amp is determined by the internal capacitances of the transistor itself (Figure 3).

If either C_{be} or C_{oB} short out at high frequencies, then the transistor stops acting as an amplifier and so the response is cut off. The values of C_{be} and C_{oB} can be found or calculated from the transistor spec sheet. Typically, C_{oB} is on the spec sheet and C_{be} is calculated from f_{T} (the gain-bandwidth product) also found on the spec sheet using: