Page 63 - Op Amps Design, Applications, and Troubleshooting

46 AMPLIFIERS

Also,

In the present case,

Vj(RMS) = vfaeak) x 0.707
= 1.07 Vx 0,707
= 756.5 mV RMS

So, for the amplifier circuit presented in Figure 2.3, input signals as great as 756.5
millivolts RMS can be amplified without saturation clipping. If you attempt to
amplify larger signals, then the peaks on the output waveform will be flattened at
the output saturation voltage limits.

Output Impedance. You will recall from Chapter 1 that the output impedance
of an op amp is generally quite low. The data sheet in Appendix 1 lists 75 ohms as
a typical output resistance for a 741 op amp. This value, however, is the open-loop
output resistance. When negative feedback is added to the amplifier (as in Figure
2.3) the effective output impedance decreases sharply. The value of effective out-
put impedance can be approximated as shown:

where A OL is the open loop gain of the op amp at the specified frequency. This can
be read from the manufacturer's graphical data (see Appendix 1) showing open-
loop gain as a function of frequency. Alternatively, you may estimate it as

where f m is the specific input frequency being considered.
For the circuit in Figure 2.3, the closed-loop output impedance can be esti-
mated at 1000 hertz as follows. First we compute the open-loop gain at 1000 hertz
by applying Equation (2.16):