TABLE OF CONTENTS It s possible to use PSPICE or other circuit simulators to determine the gain and bandwidth of transistor amplifiers, or for that matter any circuit with lumped elements and dependent sources. However, while PSPICE is a useful tool for analyzing circuits, it doesn t give that much insight and intuition into amplifier design. This chapter covers back of the envelope techniques for estimating the bandwidth of transistor amplifiers. The techniques in this chapter are employed on bipolar transistor amplifiers, but can also be used for CMOS.
The method of open-circuit time constants (OCTC) is a powerful approximate analysis tool that allows estimation of the high-frequency 3dB bandwidth of circuits containing resistors, capacitors and dependent sources. The method was developed by R. B. Adler and others at MIT. [1] and is a valuable design tool in finding the root cause of bandwidth limitation in circuits. The usefulness of the technique lies in the fact that it allows identification of local bandwidth limitations in amplifiers.
Following a brief mathematical discussion of the method of open-circuit time constants, the method is applied to several different amplifier topologies. The open-circuit time constant results are compared to closed-form solutions and SPICE simulations.
At first, let s consider a system with n real-axis poles (Figure 6-1). The transfer function for this system is:
Expanding the denominator results in:
We note that the first-order s term has as its coefficient the...
