26-09-2013, 03:31 PM
To verify the characteristics of Differential Amplifier
INTRODUCTION
Look under the hood of most op amps, comparators or audio amplifiers, and you'll discover this
powerful front-end circuit - the differential amplifier. A simple circuit able to amplify small
signals applied between its two inputs, yet reject noise signals common to both inputs. This
circuit has a unique topology: two inputs and two outputs. Although you can tap the signal from
one output only, taking the difference between both outputs delivers twice the gain! And it
improves Common-Mode Rejection (CMR), an essential function when the common-mode
signal is a noise source or DC bias from a previous stage.
GAIN AND REJECTION
How does this amplifier amplify differential signals and reject common ones? The bias condition
assumes equal voltages at VB1 and VB2, forcing the bias current IE (set by RE) to split equally
between the transistors resulting in IC1 = IC2. With RC1 = RC2, equal voltages develop at VC1
and VC2.
DIFFERENTIAL GAIN
How do we calculate the differential voltage gain? You can think of Q1 and Q2 as current
sources controlled by their base voltages. RC1 and RC2 then convert the currents back into
voltages. First, the small signal collector current
LEVEL SHIFTING
The current-source nature of the transistor's output provides a big benefit. You can shift the
output to different voltage levels. If the next stage needs a voltage biased around 25 V, for
example, simply change VCC to a higher voltage. Although the output DC bias voltage will be
higher, the gain should remain basically the same.
You can also shift the output to a negative rail! Just flip the entire circuit upside down and swap
the NPN transistors for PNPs. Many op amps and audio amps shift the output to the negative rail,
delivering the signal to the next stage, the Miller Integrator.