Page 112 - Op Amps Design, Applications, and Troubleshooting

AC-Coupled Amplifier 95

Output Voltage. The output voltage of the circuit in Figure 2.25 can be deter-
mined by utilizing the basic gain equation of Equation (2.1):

2.6.3 Practical Design Techniques

The design of a noninverting summing amplifier like that shown in Figure 2.23 is
an involved process, and the resulting design is difficult to alter without affecting
several parameters. Therefore, many designers who need a noninverting sum-
ming amplifier utilize an inverting summing amplifier followed by a simple
inverting amplifier. This arrangement is much simpler to design, easier to modify,
and costs little more to build. With this in mind, we will not explore the details for
designing the generic noninverting summing amplifier. However, we will discuss
the design of a special case that uses the same basic circuit when we study adder
circuits in Chapter 9.

2.7 AC-COUPLED AMPLIFIER

2.7.1 Operation
The term AC-coupled identifies the fact that only AC signals are allowed to pass
through the amplifier. DC and very-low-frequency AC signals are blocked or at
least severely attenuated. The concept of AC coupling is applicable to many
amplifier configurations. In the following discussion, we will consider the oper-
ation of the basic inverting and noninverting amplifier circuits when they are
configured to be AC coupled. Most of the operation, analyses, and design meth-
ods are similar to their DC-coupled equivalents, which have been covered in
detail. Therefore, we will concentrate on areas that are unique to the AC-coupled
circuit.
First let us examine the operation of the AC-coupled inverting amplifier cir-
cuit shown in Figure 2.26(a).
You will recall from basic electronics theory that a capacitor blocks EC and
passes AC. More specifically, a capacitor's opposition to current flow (capacitive
reactance) increases as the applied frequency decreases. As the input frequency in
Figure 2.26(a) decreases, the reactance of capacitors Q and C 0 both increase. As
the reactance of Q increases, the combined impedance of Q and Rj also increases.
Since the voltage gain of the inverting amplifier is determined by the ratio of the
feedback resistor to the input resistance, and since the input resistance (actually