More on the Non-inverting Amplifier

The circuit of Fig.1 has resistors R3 and R4. The amplifier would still work if R4 was removed and R3 is set to zero, but the amplifier will be more susceptible to what is known as common mode noise when the circuit is simplied to this extent. Common mode noise is defined as noise that appears in equal phase and amplitude on both inputs. This can happen if we have long traces, high impedance components, improper grounding and decoupling. Care should be taken to use short traces, the lowest practical values for resistors, proper grounding and capacitors connected to and placed close to the op-amp power pins to de-couple noise to ground. Resistor noise is proportional to the square-root of the resistance value. More on the subject of noise later.

The Non-inverting Amplifier

The non-inverting amplifier is another type of signal conditioner that instead of turning the signal upside-down simply and ideally only changes the signal amplitude and possibly shifts the DC average level of the signal (but we will not discuss level shifting at this point.)

Looking at Figure 1 you note that instead of putting the input signal into R1 as we did for the inverting amplifier, we insert it into R3 and the positive input of the amplifier (A) so the signal does not invert as it is “passed through” the amplifier. But the amplifier gain is still determined by the resistors connected to the negative terminal as before. This time the gain is positive and is

G = (Vout / Vin) = (R1 + R2) / R1

R3 and R4 are optional resistors. R3 can be zero and R4 can be open (or infinite value – not in the circuit.) R4 can also be leakage resistance to ground. If R3 and R4 are used then there is a voltage divider effect on the signal so that the actual positive terminal input signal is

Vin’ = Vin * R4 / (R3 + R4)

Then we have an additional gain factor of

G’ = R4 / (R3 + R4)

and the overall gain will be

Gtotal = G’*G

If R3 and R4 are used they can be any convenient values but if you set R3 = R1 and R4 = R2, then you have a balanced impedance input on each terminal of the op-amp. We will discuss this further later. The power supply connections are not shown for the sake of simplicity.

As with the inverting op-amp circuit, the current into the summing point (the negative input) is again zero by definition. Also the current into the positive terminal is zero by definition of the ideal op-amp. This requires the following equations to be true:

Vout = I * (R1 + R2)

where

I = Iin = Ifb

and Iin is the “input current” through R1 and Ifb is the “feedback current” through R2, and

Iin = Vin / R1

So we now have the formula for the output voltage of the op-amp as


Vout = (Vin / R1 ) * (R1 + R2)

which is the same as

Vout = G * Vin

The inverting amplifier can be thought of as some type of signal conditioner that turns the input signal up-side down or inverts the signal. An easy way to build one is with an IC called an op-amp, short for operational amplifier. Op-amps have some seemingly peculiar characteristics which we will mention later. Fig. 1 shows a simple schematic of a basic op-amp inverting amplifier.

The power supply connections are not shown in Fig. 1 for the sake of simplicity. Some basic facts about the op-amp are as follows (some of the pecularities):

1. The negative input of the op-amp (the – terminal of A) is a virtual ground or zero potential relative to Vref (virtually ground if Vref is ground) and is sometimes referred to as the “summing point” as we shall later. This means that the current from V in through the resistor R1 is

I in = V in / R1

and

2.
I_f = -V out / R2

where I_f is the “feedback current” through R2. The feedback current has to be the negative of the input current so that the sum is zero and the current into the negative terminal of A is zero by definition. The negative sign indicates the inversion of the signal.

By doing a little algebra we can combine the above two equations and arrive at the basic equation for gain of the inverting amplifier:

G = (Vout / Vin) = - R2 / R1

again the negative sign in the gain equation (sometimes referred to as the “transfer function”) indicates the inversion.

What is this blog about?

This blog will cover various topics in analog electronics including circuits, parts, and theory from elementary to medium difficulty. The purpose will be to educate, provide information on design, techniques, tricks, simulation, and maybe a little entertainment thrown in.

Reader's comments are welcome. Good postings will be accepted as time permits.

Ed Boeckmann 10-28-2007