Page 58 - Op Amps Design, Applications, and Troubleshooting

Inverting Amplifier 41

When the feedback and input resistor combination in Figure 2.2 are viewed
as an unloaded voltage divider, it is easy to determine current flow. Since we
know that no significant current is allowed to enter or leave the (-) pin of the op
amp, we can conclude that any current flowing through R/ must also pass through
R F. The polarity of the input and output voltages will determine the direction of
this current, but it is important to realize that the value of current through R r is the
same as the current through R F.
Since very little current flows in or out of the input terminals of the op amp,
we saw that there was essentially no voltage drop across R B which caused the (+)
input terminal to remain at ground potential. Since % is always near 0 as long as
the amplifier remains unsaturated, this means that the (-) input terminal must
also remain very near to ground potential. This is an important concept. Although
the (-) input is not actually grounded, it remains very near ground potential. We
commonly refer to this point in the circuit as virtual ground.
R B is included to compensate for errors caused by the fact that some bias cur-
rent does flow in or out of the op amp terminals. Even though this bias current is
small, it can cause a slight voltage drop across R F and Rj. This voltage drop is then
amplified and appears at the output of the op amp as an error voltage. By includ-
ing R B in series with the (+) terminal and making its value equal to the parallel
combination of RF and R fr we can generate a voltage that is roughly equal, but the
opposite polarity from that caused by the drop across Rp and R/. The error is gen-
erally reduced substantially but will be reduced to 0 only if the bias currents in the
two input terminals happen to be equal. We will discuss this in greater detail in
Chapter 10,

2.2.2 Numerical Analysis
Let us now learn to analyze the performance of the inverting amplifier circuit. We
will calculate all of the following:

1. Voltage gain
2. Input impedance
3. Input current requirement
4. Slew-rate limiting frequency
5. Maximum output voltage swing
6. Maximum input voltage swing
7. Output impedance
8. Output current capability
9. Minimum value of load resistance
10. Bandwidth
11. Power supply rejection ratio

For purposes of our first numerical analysis exercise, let us evaluate the perfor-
mance of the circuit in Figure 2.3.