Frequency Response of Multi-Transistor Amplifiers

The OCTCA becomes especially useful for multi-transistor circuits where the number of loop equations quickly becomes too numerous for conventional analysis. As an example, consider the typical circuit which contains a CE gain stage followed by an EF buffer stage shown in Fig. 4.16.

  

Figure 4.16: Typical Two-Transistor Amp

This circuit has the following high frequency equivalent shown in Fig. 4.17.

  

Figure 4.17: High Frequency Equivalent Circuit of CE-EF Amp

Using the OCTCA techniques, we can approximate the dominant pole frequency, and obtain a fairly accurate description of the high-frequency roll-off of the circuit. All we have to do is find the resistance seen by each capacitor individually. The dominant pole is then approximated by

$$p=\frac{-1}{C_{\pi 1}R_{\pi 1}+ C_{\mu 1}R_{\mu 1} +C_{\pi 2}R_{\pi 1}+C_{\mu 2}R_{\mu 2}}$$ (94)

Where

$$R_{\pi 1}=R_S\vert\vert R_{in1}$$ (95)

$$R_{\mu 1}=R_{\pi 1}+R_{L1}+g_mR_{L1}R_{\pi 1},$$ (96)

$$R_{L1}=R_C\vert\vert(r_{\pi 2}+\beta R_{E2})=R_{in2}$$ (97)

$$R_{\pi 2}=r_{\pi 2}\vert\vert\frac{R_C+R_{E2}}{1+g_{m2}R_{E2}}$$ (98)

$$R_{\mu 2}=R_C\vert\vert R_{in2}$$ (99)