Differential Amplifier using Transistors

[jeremycool]

As the name indicates Differential Amplifier is a dc-coupled amplifier that amplifies the difference between two input signals. It is the building block of analog integrated circuits and [from admin: url removed, stealth advertising detected] (op-amp). One of the important feature of differential amplifier is that it tends to reject or nullify the part of input signals which is common to both inputs. This provides very good noise immunity in a lot of applications. Let’s see the block diagram of a differential amplifier.



Vi1 and Vi2 are input terminals and Vo1 and Vo2 are output terminals with respect to ground. We can feed two input signals at the same time or one at a time. In the former case it is called dual input otherwise it is single input. Similarly there are two ways to take output also. If the output is taken from one terminal with respect to ground, it is unbalanced output or if the output is taken between two output terminals, it is balanced output.

Differential Amplifier using BJT

The simplest form of differential amplifier can be constructed using Bipolar Junction Transistors as shown in the below circuit diagram. It is constructed using two matching transistors in common emitter configuration whose emitters are tied together.


Configurations

Based on the methods of providing input and taking output, differential amplifiers can have four different configurations as below.
Single Input Unbalanced Output
Single Input Balanced Output
Dual Input Unbalanced Output
Dual Input Balanced Output
Single Input Unbalanced Output

In this case, only one input signal is given and the output is taken from only one of the two collectors with respect to ground as shown below.

When input signal Vin1 is applied to the transistor Q1, it’s amplified and inverted voltage gets generated at the collector of the transistor Q1. At the same time it’s amplified and non-inverted voltage gets generated at the collector of the transistor Q2 as shown in the above diagram. Unbalanced output will contain unnecessary dc content as it is a dc coupled amplifier therefore this configuration should follow by a level translator circuit.

How the transistor Q2 also producing output voltage even though the input is provided only to transistor Q1 ?

The effect of input voltage Vin1 is coupled to the transistor Q2 via the common emitter resistor RE.

[neonlamp]

You said the answer yourself. If Q2 base is grounded, and Q1 base sees an input signal then its emitter will try to follow the base voltage. If it rises, the transistor Q1 takes more of the current from the tail resistor RE. That means Q2 has less current. That means its collector voltage will rise. In phase with the input, in other words.

[SteveMJ]

The resistor Re is common to bother sides of the differential circuit. Current flowing through it from Q1 will affect the volt drop across it, which in turn affect the emitter voltage of Q2.

Hope this helps

[Marrs]

[admin] link removed.