# Why would we want a negative voltage as an output

In the problem, I solved that the transfer function $$H(s) = \frac{V_{out}}{V_{in}} = -\frac{Z_f}{Z_i}$$

My professor told me that the op-amp circuit allows for a transfer function whose voltage output isn't affected by its load. However, what purpose would a negative voltage have if we are trying to get a strong and steady voltage?

• Perhaps you want a strong and steady negative voltage? Commented Nov 23, 2018 at 15:35
• Or your input maybe negative.... Commented Nov 23, 2018 at 15:35
– Chu
Commented Nov 24, 2018 at 2:12

You have an inverting amplifier, which means your gain is negative.

$$A_V = \frac {V_{out}} {V_{in}} = - \frac {R_F} {R_2}$$

If you want positive gain, use a non-inverting amplifier.

$$A_V = \frac {V_{out}} {V_{in}} = 1 + \frac {R_F} {R_2}$$

An op-amp is a directly-coupled amplifier, so it will amplify ac or DC signals.

If it is a sinewave, the negative gain of inverting amplifier means the output will be inverted 180°. It is still a sinewave.

Put it through a second inverting amplifier and the negatives will cancel out. If you are dealing with audio, you will need several stages to match input impedance of source (antenna - μW) and output impedance of load (8Ω speaker - W).

As a DC input signal, the output will be inverted. So a positive signal will make a negative signal and vice versa.

Positive or negative levels are just potentials. As Felthry says "Perhaps you want a strong and steady negative voltage."

• It isn't phase shifted for non-sinusoidal input signals; it's inverted and that should be the terminology that is used whether it's a sine wave or some other signal. Commented Nov 23, 2018 at 18:53
• @Andyaka Good point. Commented Nov 23, 2018 at 19:34

A feedback technique is designed to control gain of an opamp. Coming to non-inverting configuration we achieve a gain greater than 1,so in order to achieve any required gain(maybe less than 1) we prefer inverting configuration.(even though the output signal is inverted)

My professor told me that the op-amp circuit allows for a transfer function whose voltage output isn't affected by its load.

Yes that is correct.

However, what purpose would a negative voltage have if we are trying to get a strong and steady voltage?

In many applications, and particularly in audio applications, it doesn't matter whether an amplifier inverts or not. Your ear will not be able to tell the difference unless there is a reference signal with which to compare.

The inverting op-amp configuration you have in your question has useful features that might make it a good choice if inversion is not a problem.

1. The inverting input is at "virtual-ground" as shown in your annotations. This means that the output will be determined by the sum of input currents and so a very simple summing amplifier can be made (again with inversion).

Figure 1. Summing inverting amplifer. Source Wikibooks.

1. It can amplify by a factor < 1. This is useful with high-voltage inputs, for example.

See the linked article for more.