Question

The non-inverting amplifier has a:
A. Large closed-loop voltage gain
B. Small open-loop voltage gain
C. Large closed-loop input impedance
D. Large closed-loop output impedance

Answer 1

Recall that in a non-inverting amplifier, the input is fed into the positive terminal of the operational amplifier such that the polarity of the output signal remains the same as the input signal. We also know that for a closed-loop amplifier in the non-inverting condition, the voltage gain is related to the feedback and the input resistances. Use this relation to deduce the suitable values of the input and output impedance to ensure a gain of at least unity. Keep in mind that open-loop configurations have no feedback mechanisms.
Formula Used:
Closed-loop voltage gain $A_{v} = 1 + \dfrac{R_f}{R_{in}}$

Complete step by step answer:
We know that a non-inverting amplifier is an operational amplifier whose output signal has the same polarity as the input signal. An operational amplifier has two terminals, one with a minus (-) sign and the other with a plus (+) sign. When we apply a signal to the (+) sign terminal, the op amp retains the original polarity of the input signal when it gets amplified at the output terminal. Such a configuration is called a non-inverting amplifier.
The measure of how much an amplifier ‘amplifies’ a signal is given by the term gain and is usually associated with the input and output signal voltages.
Now, the amplifier is said to have an open loop configuration when no feedback in any form is fed back to the input, and in such a configuration, the open loop voltage gain is usually set to infinity since any voltage differential between the two input terminals will result in an infinite voltage at the output.
The amplifier is said to have a closed loop configuration when some of the output is fed back into the input. If $R_{f}$ is the feedback resistance and $R_{in}$ is the input impedance, then the voltage gain in such a configuration is given by:
$A_{v} = \dfrac{V_{out}}{V_{in}} = 1 + \dfrac{R_f}{R_{in}}$
Now, when the input impedance is large, i.e., $R_{in} \rightarrow \infty$, the above equation becomes:
$A_{v} = 1 + \dfrac{R_f}{\infty} = 1 + 0 = 1$
The feedback impedance, however, is usually kept to a minimum or zero to obtain efficient amplification by usually shorting it and directly connecting the output to the input to ensure a full feedback.
Thus, for a suitable gain, the non-inverting amplifier has a large closed-loop input impedance.

So, the correct answer is “Option C”.

Note: We saw that the closed-loop voltage gain of a non-inverting amplifier is given as:
$A_{v} = 1 + \dfrac{R_f}{R_{in}}$
Whereas, the closed-loop gain for an inverting amplifier is given as:
$A_v = -\dfrac{R_f}{R_{in}}$
The non-inverting gain expression leaves no room for any negative values, thus proving that the input signal to the circuit gets amplified without changing its polarity at the output.
Note that those amplifier circuits with shorted feedback resistance are also called unity gain buffers. This means that the output voltage is the same as the input voltage, and is used to isolate different stages of a circuit from one another. They are of two types: voltage followers, where the output has exactly the same voltage as the input, and voltage inverters, where the output has the same voltage level as the input but with opposite polarity.