Op-amp can add more than two voltages, while discrete transistors can't?

Question

One of the application circuits for the op-amp is the summing amplifier which can add as many input signals as you need.

It was mentioned that this is one of the advantages of the op-amp over discrete transistors, as the only circuit with discrete transistors that can add two signals is the differential amplifier with the second input signal reversed before passed to the differential amplifier.

My question is, how does the op-amp add more than two input signals when it is actually made of discrete transistors circuits?

Am I missing something here?

Answer 1

A transistor can add n input signals, a crude example is the following

^{simulate this circuit – Schematic created using CircuitLab}

You can also do it with a common base circuit.

Answer 2

the only circuit with discrete transistors that can add two signals is the differential amplifier with the second input signal reversed before passed to the differential amplifier.

That's simply not true.

My question is, how does the op-amp add more than two input signals when it is actually made of discrete transistors circuits?

Well, a) above statement is false, b) the opamp is a differential amplifier (with a very high gain).

How the summing amplifier built from an opamp works is explained in 1000 places on the internet, so I'll allow you to research that yourself.

Answer 3

As their name suggests, devices with differential input can subtract two single-ended (grounded) voltages. The transistor is a simple example where the first input voltage is applied between the base and ground and the second voltage - between the emitter and ground. The differential amplifier, particularly an op-amp, is a more sophisticated example where the first voltage is applied between the non-inverting input and ground... and the second voltage is applied between the inverting input and ground.

Actually, in these configurations, the two input voltages are connected in series and opposite (traveling the input loop). So we can say they are summed in a series manner, according to KVL, by the input loop… and the output voltage appears at the differential input. The conclusion is that the humble loop acts as the simplest series voltage summer.

We can enlarge this idea adding/subtracting more input voltage sources by connecting them in series, in direct/opposite direction - Fig. 1. But a problem appears - only two of them can be grounded; others will be floating (ungrounded).

Fig. 1. Summing voltages in a series manner (according to KVL)

This problem can be solved if, instead of summing voltages in series (according to KVL), we sum currents in parallel (according to KCL); thus all input current sources can be grounded - Fig. 2. So the current summer is nothing else than a humble node. It remains only to convert the input voltages to proportional currents by resistors in series (according to Ohm's law) and we will obtain the parallel resistor summer shown in the @S.s. answer.

Fig. 2. Building a parallel resistor summer (according to KCL and Ohm's law)

So, the general conclusion is:

Addition and subtraction are performed by simple electrical circuits and not by active elements (transistors and op-amps). The role of active elements is only to make the passive summing circuits perfect.

For example, in the op-amp inverting summer, the "feedback resistor" R is just another (fourth) input where the op-amp output voltage is applied with such a polarity so that to zero (compensate) the voltage of the common summing point A; as a result, it becomes a "virtual ground". Thus the input currents depend only on the corresponding input voltages and the inputs are independent of each other (they are separated by the virtual ground).

Fig. 3. Op-amp inverting summer

See also my Wikibooks stories about the series and parallel summer.

Answer 4

The short answer is that an opamp has way more going on inside than just the input differential pair. If you recreate the rest of the opamp's internal functions with discrete transistor circuits, you will get the same results.