The operational amplifier in inverting and non-inverting configurations and their gain equations, the summing amplifier (mixer) that adds several inputs, and the amplifier system block diagram from signal source to loudspeaker.

What this topic is asking

WJEC Eduqas wants you to know the operational amplifier (op-amp) in its inverting and non-inverting configurations and their gain equations, the summing amplifier (mixer) that adds several inputs together, and the block diagram of a complete amplifier system from a signal source (such as a microphone) through to a loudspeaker.

The operational amplifier

The op-amp is the standard amplifying building block. Its huge open-loop gain is tamed by feeding some of the output back to the inverting input through a resistor: this negative feedback fixes the gain at a value set only by the external resistors, which is stable and predictable. By choosing the resistor values and which input the signal enters, you make an inverting or a non-inverting amplifier.

The inverting amplifier

In the inverting amplifier the signal is applied through $R_{\text{in}}$ to the inverting ($-$) input, with $R_f$ feeding the output back to the same point and the non-inverting input held at $0\,\text{V}$. The gain is just the ratio of the two resistors, with a minus sign because the output goes the opposite way to the input. For example, $R_f = 100\,\text{k}\Omega$ and $R_{\text{in}} = 10\,\text{k}\Omega$ give a gain of $-10$. Changing the resistors changes the gain.

The non-inverting amplifier

In the non-inverting amplifier the signal is applied directly to the non-inverting ($+$) input, while $R_f$ and $R_{\text{in}}$ set the feedback. The output is in phase with the input, and because of the "$1 +$" the gain is always at least 1. For example, $R_f = 100\,\text{k}\Omega$ and $R_{\text{in}} = 10\,\text{k}\Omega$ give a gain of $1 + 10 = 11$. Choose inverting or non-inverting depending on whether the application can tolerate the signal being turned upside down.

The summing amplifier (mixer)

The summing amplifier exploits the fact that the inverting input acts as a common point: each input contributes a current through its resistor, and the amplifier adds them, producing an output equal to the (weighted, inverted) sum. With equal input resistors each signal is added equally; with different resistors each input can be given a different weight (volume). This is exactly what a mixing desk does: several microphones or instruments are summed into one output.

The amplifier system

No single stage does everything: the microphone signal is tiny, so a pre-amplifier raises its voltage; a mixer combines sources and sets the volume; and a power amplifier provides the large current needed to drive a loudspeaker, which the small-signal stages cannot. Drawing the block diagram, left to right with arrows, is the systems approach applied to audio, and a common exam task.

Try this

Q1. An inverting amplifier has $R_f = 47\,\text{k}\Omega$ and $R_{\text{in}} = 4.7\,\text{k}\Omega$. Calculate its gain. [2 marks]

• Cue. $G = -\dfrac{47}{4.7} = -10$.

Q2. State the configuration to use if the output must stay the same way up as the input. [1 mark]

• Cue. A non-inverting amplifier.