In This Chapter

Various and sundry issues related to advanced amplifier design techniques are discussed.

Worst-Case Open-Circuit Time Constant Calculations

The most complicated open-circuit time constant calculation is found for a transistor amplifier that has a source resistance ( R _s), emitter resistance ( R _E) and collector load resistance ( R _L), as shown in Figure 7-1. In the following section, we ll figure out the open-circuit resistances for C _? and C _? for this worst-case circuit.

Figure 7-1: Worstcase open-circuit time constant circuit with source resistance R _s, emitter resistor R _E and collector load resistor R _L.

The worst-case open-circuit time constants circuit for C _? is shown in Figure 7-2a. The mathematics rapidly gets difficult if we blindly begin writing node equations. First, let s recognize that the test voltage source v _t sets the value of v _?. Since the v _t generator sets the value of the dependent ^[1] current source, we can use superposition to find the test current.

Figure 7-2: Circuits for finding worst-case OCTC for C _?. (a) Original circuit. (b) Circuit for finding contribution due to test voltage source v _t only. (c) Circuit for finding contribution due to g _mv _t generator only.

Using the circuit of Figure 7-2b, we ll find the test current due to the test voltage source, with the g _m v _t dependent generator set to zero...