**Direct Coupled Inverting Amplifier:**

The circuit in Fig. 14-18 is termed an Direct Coupled Inverting Amplifier because, with V_{i} applied via R_{1} to the inverting input terminal, the output goes negative when the input goes positive, and vice versa. Note that the noninverting input terminal is grounded via resistor R_{3}. With the noninverting terminal grounded, the voltage at the op-amp inverting input terminal remains close to ground. Any increase or decrease in the (very small) difference voltage between the two input terminals is amplified by the op-amp open-loop gain and fed back via R_{2} and R_{1} to correct the change. Because the inverting input terminal is not grounded but remains close to ground, the inverting input terminal in this application is termed a **virtual ground** or **virtual earth**.

The circuit input current in Fig. 14-18 can be calculated as,

The input voltage is applied to one end of R_{1}, and the other end of R_{1} is at ground level. Consequently,

I_{1} is always selected to be very much larger than the op-amp input bias current (I_{B}). Consequently, virtually all of I_{1} flows through resistor R_{2}, (see Fig.14-18), and the voltage drop across R_{2} is,

The left side of R_{2} is connected to the op-amp inverting input terminal, which, as discussed, is always at ground level. This means that the right side of R_{2} (the output terminal) is V_{R2} below ground. So,

From Eq. 14-10, V_{i} = I_{1}R_{1}

So, the circuit voltage gain is,

If the input of the inverting amplifier is grounded, the circuit is seen to be exactly the same as noninverting amplifier with R_{3} as the input resistor and zero input voltage. Thus, negative feedback occurs (as in a noninverting amplifier) to maintain the op-amp inverting input terminal at the same voltage level as the noninverting input terminal.

The output impedance of an inverting amplifier is calculated in exactly the same way as for a noninverting amplifier, (using Eq. 13-6). Like the case of the noninverting amplifier, the output impedance is very low for an inverting amplifier.

The input impedance of an inverting amplifier is easily determined by recalling that the right side of R_{1} (in Fig. 14-18) is always at ground level, and that the signal is applied to the left side of R_{1}. Therefore,

Design of an inverting amplifier is very simple. Voltage divider current I_{1} is selected very much larger than the op-amp input bias current. Resistors R_{1} and R_{2} are calculated from Eqs. 14-10 and 14-11, or 14-12, and R_{3} is selected approximately equal to R_{1}||R_{2}.

**Capacitor-Coupled Inverting Amplifier:**

A capacitor-coupled inverting amplifier circuit is shown in Fig. 14-20. In this case, the bias current to the op-amp inverting input terminal flows (from the output) via resistor R_{2}, so that the input coupling capacitor does not interrupt the input bias current. A voltage drop I_{B}R_{2} is produced at the inverting input by the bias current flow, and this must be equalized by the I_{B}R_{3} voltage drop at the noninverting input. ‘Therefore, for the capacitor-coupled noninverting circuit,

Apart from the selection of R_{3}, the circuit is designed exactly as the Direct Coupled Inverting Amplifier, and the capacitor values are calculated in the same way as for a capacitor-coupled noninverting amplifier.