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# Differential Amplifiers

A differential amplifier is an amplifier that can have two input signals and two output signals. This arrangement means that the differential amplifier can be used in a variety of ways. The differential amplifier can amplify the difference between two input signals. A differential amplifier will also "cancel out" common signals at the two inputs.

## Typical differential amplifier circuit

The figure below is the schematic diagram of a typical differential amplifier.
Notice that there are two inputs and two outputs. This circuit requires
two bipolar transistors to provide the two inputs and two outputs. If
you look at one input and the transistor with which it is associated, you
will see that each transistor is a common-emitter amplifier for that input
(input one and *Q*_{1}; input two and *Q*_{2}).
*R*_{1} develops the signal at
input one for *Q*_{1}, and *R*_{5} develops the
signal at input two for *Q*_{2}. *R*_{3} is the
emitter resistor for both *Q*_{1} and *Q*_{2}. Notice
that *R*_{3} is NOT bypassed. This means that when a signal
at input one affects the current through *Q*_{1}, that signal
is developed by *R*_{3}. (The current through
*Q*_{1} must flow through *R*_{3}; as this
current changes, the voltage developed across *R*_{3} changes.)
When a signal is developed by *R*_{3}, it is
applied to the emitter of *Q*_{2}. In the same way, signals
at input two affect the current of *Q*_{2}, are developed
by *R*_{3}, and are felt on the emitter of
*Q*_{1}. *R*_{2} develops the signal
for output one, and *R*_{4} develops the signal for output two.

Even though this circuit is designed to have two inputs and two outputs, it is not necessary to use both inputs and both outputs. (Remember, a differential amplifier was defined as having two possible inputs and two possible outputs.) A differential amplifier can be connected as a single-input, single-output device; a single-input, differential-output device; or a differential-input, differential-output device.

## Single-input, single-output differential amplifier

The figure below shows a differential amplifier with one input (the base of
*Q*_{1}) and one output (the collector of *Q*_{2}).
The second input (the base of *Q*_{2}) is grounded and the second
output (the collector of *Q*_{1}) is not used.

When the input signal developed by *R*_{1} goes positive, the current through
*Q*_{1} increases. This increased current causes a positive-going
signal at the top of *R*_{3}. This signal is felt on the emitter
of *Q*_{2}. Since the base of *Q*_{2} is grounded,
the current through *Q*_{2} decreases with a positive-going
signal on the emitter. This decreased current causes less voltage drop across
*R*_{4}. Therefore, the voltage at the bottom of
*R*_{4} increases and a positive-going signal is felt at the output.

When the input signal developed by *R*_{1} goes negative, the current through
*Q*_{1} decreases. This decreased current causes a negative-going
signal at the top of *R*_{3}. This signal is felt on the
emitter of *Q*_{2}. When the emitter of *Q*_{2} goes
negative, the current through *Q*_{2} increases. This increased
current causes more of a voltage drop across *R*_{4}. Therefore,
the voltage at the bottom of *R*_{4} decreases and a negative-
going signal is felt at the output.

This single-input, single-output, differential amplifier is very similar to a single-transistor amplifier as far as input and output signals are concerned. This use of a differential amplifier does provide amplification of AC or DC signals but does not take full advantage of the characteristics of a differential amplifier.

## Single-input, differential-output, differential amplifier

The figure below shows a differential amplifier with one input (the base of
*Q*_{1}) and two outputs (the collectors of
*Q*_{1} and *Q*_{2}). One output is in phase with
the input signal, and the other output is 180 degrees
out of phase with the input signal. The outputs are differential outputs.

This circuitâ€™s operation is the same as for the single-input, single-output
differential amplifier just described. However, another output is obtained
from the bottom of *R*_{2}. As the input signal goes positive,
thus causing increased current through *Q*_{1}, *R*_{2}
has a greater voltage drop. The output signal at the bottom
of *R*_{2} therefore is negative going. A negative-going input
signal will decrease current and reverse the polarities of both output signals.

Now you see how a differential amplifier can produce two amplified,
differential output signals from a single-input signal. One further point of
interest about this configuration is that if a combined output
signal is taken between outputs number one and two, this single output will
be twice the amplitude of the individual outputs. In other words, you can
double the gain of the differential amplifier (single output) by
taking the output signal between the two output terminals. This single-output
signal will be in phase with the input signal. This is shown by the phantom
signal above *R*_{5} (the phantom resistor connected between
outputs number one and two would be used to develop this signal).

## Differential-input, differential-output, differential amplifier

When a differential amplifier is connected with a differential input and a differential output, the full potential of the circuit is used. The figure below shows a differential amplifier with this type of configuration (differential-input, differential-output).

Normally, this configuration uses two input signals that are 180 degrees out of phase. This causes the difference (differential) signal to be twice as large as either input alone. (This is just like the two-input, single-output difference amplifier with input signals that are 180 degrees out of phase.)

Output number one is a signal that is in phase with input number two, and output number two is a signal that is in phase with input number one. The amplitude of each output signal is the input signal multiplied by the gain of the amplifier. With 180-degree-out-of-phase input signals, each output signal is greater in amplitude than either input signal by a factor of the gain of the amplifier.

When an output signal is taken between the two output terminals of the amplifier (as shown by the phantom connections, resistor, and signal), the combined output signal is twice as great in amplitude as either signal at output number one or output number two. (This is because output number one and output number two are 180 degrees out of phase with each other.) When the input signals are 180 degrees out of phase, the amplitude of the combined output signal is equal to the amplitude of one input signal multiplied by two times the gain of the amplifier.

When the input signals are not 180 degrees out of phase, the combined output signal taken across output one and output two is similar to the output that you were shown for the two-input, single-output, difference amplifier. The differential amplifier can have two outputs (180 degrees out of phase with each other), or the outputs can be combined as shown in the figure above.