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## Differential Amplifiers

The differential amplifier amplifies the difference between two input signals (-) and (+). This amplifier is also referred to as a differential-input single-ended output amplifier. It is a precision voltage difference amplifier, and forms the central basis of more sophisticated instrumentation amplifier circuits.

A differential amplifier is shown in figure 6.18. The voltage is given by (cf. voltage divider)

Figure 6.18:  Differential amplifier.

and thus

The differential amplifier is usually limited in its performance by the low input impedance of . Two buffer amplifiers are commonly added to remove this limitation and form the simple instrumentation amplifier in figure 6.19.

Figure 6.19:  Instrumentation amplifier.

Example: If the open-loop gain curve in figure 6.20 describes the amplifier shown, write an expression for when mV.

Figure 6.20:  Open loop gain curve and amplifier.

By definition

Since is at ground and , we have .

Since .

Therefore

and

Example: The variable resistor in the circuit in figure 6.21 provides a DC offset to the output voltage; a circuit of this type is often used to provide a zero output for some specific but nonzero input signal. What range of input voltage can be zeroed out by this circuit?

Figure 6.21:  Circuit with DC offset.

The current flowing from the input to the output is

Therefore

ranges from 0 to .

Therefore ranges from to .

For to or  V.

The range of input singals that can be zeroed is

Example: A two-input current summing amplifier can be used to shift the DC level of an AC signal. For the circuit shown below, determine the average value of the output signal if the input is V.

Figure 6.22: Two-input current summing amplifier.

The current sum into the node is

since is at virtual ground ( ).

since .

Therefore .

Example: Assuming ideal operational amplifiers, determine for each of the circuits below. Then determine the single-term approximations to the transfer function at various frequencies, sketch the resulting straight-line approximation to , and label all magnitudes, corners and slopes.

Figure 6.23: Amplifiers with a feedback capacitor.

a)
Since is at virtual ground

where

; .

; .

.

Figure 6.24: Straight-line approximation to for amplifier a).

b)

Therefore

; .

; .

.

Figure 6.25: Straight-line approximation to for amplifier b).

c)

Therefore

; .

; .

.

Figure 6.26: Straight-line approximation to for amplifier c).

d)

Therefore

; .

; .

.

Figure 6.27: Straight-line approximation to for amplifier d).

Next: Analysis Using Finite Open-Loop Up: Ideal Amplifier Approximation Previous: General Feedback Elements

Doug Gingrich
Tue Jul 13 16:55:15 EDT 1999