Page 80 - Op Amps Design, Applications, and Troubleshooting

Noninverting Amplifier U3

where R OP is the value of input resistance provided by the manufacturer and A v is
the open-loop voltage gain of the op amp. For the circuit shown in Figure 2.12, we
can estimate input resistance at low frequencies as

= 9296 MO

If we had used the more typical value of 2.0 megohms for the op amp resis-
tance (Rop), we would have gotten a much higher value for input resistance. In
either case, the actual effective input resistance is extremely high. This high input
resistance is one of the primary advantages of the noninverting amplifier in many
applications.

Input Current Requirement. The input current can be estimated by applying
Ohm's Law to the input circuit as follows:

Even this is a worst-case value. If we had used the higher typical value for input
resistance, we would have computed an even smaller value. For many, if not most,
applications, this input current can be considered negligible. If it becomes neces-
sary to consider this current, then additional considerations must be made because
the exact value of input resistance varies considerably with temperature and fre-
quency.

Maximum Output Voltage Swing. As we found with the inverting ampli-
fier, the output voltage of an op amp is limited by the ± V SAT levels. For most appli-
cations utilizing a bipolar op amp, the saturation voltages can be estimated at
about 2 volts less than the DC supply voltage. In the case of Figure 2.12, we com-
pute the maximum output swing, Equation (2.10), as

If a more accurate value is desired, the manufacturer's data sheet can be used to
find a more precise value for the worst-case saturation voltage.