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Understanding the mechanisms involved in a voltage amplifier

In our previous articles, we discussed analogue signals and their characteristics. We’re going to dive a little deeper into analogue signals and see how a voltage amplifier works.

What is an operational amplifier?

Operational amplifiers, or op-amps, are one of the basic building blocks of analogue electrical circuits. They are linear devices that have all the properties for almost ideal DC amplification. This means they’re used in signal conditioning, filtering or to perform mathematical operations.

An operational amplifier is fundamentally a voltage amplifying device. It’s designed to be used with external feedback components, such as resistors and capacitors, between its output and input terminals.

The function of the amplifier is determined by the feedback components and the different feedback considerations, such as resistive, capacitive or both. The amplifier can perform a variety of different operations, which is where it gets the name ‘operational amplifier’.

It’s a three-terminal device which consists of two high impedance inputs.

One of the inputs is called the Inverting Input, marked with a negative or “minus” sign, ( – ).
Another input is called the Non-inverting Input, marked with a positive or “plus” sign ( + ).
The third terminal represents the operational amplifier’s output port, which can both sink and source either a voltage or a current.

How does it work?

In a linear operational amplifier, the output signal is the amplification factor, known as the amplifier’s gain (A). It’s multiplied by the value of the input signal, and depending on the nature of these input and output signals, there can be four different classifications of operational amplifier gain:

Voltage: voltage “in” and voltage “out”
Current: current “in” and current “out”
Transconductance: voltage “in” and current “out”
Transresistance: current “in” and voltage “out”

, Understanding the mechanisms involved in a voltage amplifier

The output voltage signal from an Operational Amplifier is the difference between the signals being applied to its two individual inputs and is therefore known as a differential amplifier.

The circuit in the image above shows a generalised form of a differential amplifier with two inputs marked V₁ and V₂. The two identical transistors TR₁ and TR₂ are both biased at the same operating point with their emitters connected together and returned to the common rail, -V_ee by way of resistor R_e.

It operates from a dual supply +V_cc and -V_ee which ensures a constant supply. The voltage that appears at the output, V_out of the amplifier is the difference between the two input signals as the two base inputs are in anti-phase with each other.

V_out = gain(V₊ – V_–)

So as the forward bias of transistor, TR₁ is increased, the forward bias of transistor TR₂ is reduced and vice versa. If the two transistors are perfectly matched, the current flowing through the common emitter resistor, R_e will remain constant.

Both the input signal and the output signal are balanced, and since the collector voltages either swing in opposite directions (anti-phase) or in the same direction (in-phase), the output voltage signal, taken from between the two collectors, assumes a perfectly balanced circuit with a zero difference between the two collector voltages.This is known as the Common Mode of Operation with the common mode gain of the amplifier being the output gain when the input is zero.

Basic Operational Amplifiers have one output of low impedance that is referenced to a common ground terminal, and it should ignore any common mode signals, that is, if an identical signal is applied to both the inverting and non-inverting inputs, there should be a change to the output.

However, in real amplifiers, there is always some variation, and the ratio of the change to the output voltage with regards to the change in the common mode input voltage is called the Common Mode Rejection Ratio, or CMRR for short.

Operational Amplifiers on their own have a very high open-loop (no feedback) DC gain, and by applying some form of Negative Feedback, we can produce an operational amplifier circuit that has a very precise gain characteristic that is dependent only on the feedback used.

An operational amplifier only responds to the difference between the voltages on its two input terminals, known commonly as the “Differential Input Voltage” and not to their common potential. Then, if the same voltage potential is applied to both terminals, the resultant output will be zero. An Operational Amplifiers gain is commonly known as the Open Loop Differential Gain, and is given the symbol (A_o).

Keep an eye out for our next articles, where we dive even further into analogue and digital electronics.