High-Frequency Amplifier Response (bjt)

Now that we've got a high frequency models for the BJT, we can analyze the high frequency response of our basic amplifier configurations. Note: in the circuits that follow, the actual signal source (v S) and its associated source resistance (R S) have been included. As discussed in the low frequency response section of our studies, we always knew that this source and resistance was there but we just started our investigations with the input to the transistor (v in). Remember-the analysis process is the same and the relationship between v S and v in is a voltage divider! Again, in some instances in the following discussion, I will be using slightly different notation and taking a different approach than your author. As usual, if this results in confusion, or you are more comfortable with his technique, let me know and we'll work it out. High Frequency Response of the CE and ER Amplifier The generic common-emitter amplifier circuit of Section D2 is reproduced to the left below and the small signal circuit using the high frequency BJT model is given below right (based on Figures 10.17a and 10.17b of your text). Note that all external capacitors are assumed to be short circuits at high frequencies and are not present in the high frequency equivalent circuit (since the external capacitors are large when compared to the internal capacitances – recall that Z c =1/jωC gets small as the frequency or capacitance gets large). We can simplify the small signal circuit by making the following observations and approximations: C ce is very small and may be neglected. r ce >> (R C ||R L), so r ce may be neglected since r ce ||R C ||R L is dominated by R C ||R L .