SINGLE-SUPPLY INVERTING AMPLIFIER

There may be cases where the input signal going into the

amplifier is negative. Because the amplifier is operating in

single supply voltage, a voltage divider using R

implemented to bias the amplifier so the inverting input

signal is within the input common voltage range of the am-

plifier. The capacitor C

and resistor R

signal source, V

π R

centered around mid-supply (if the voltage divider provides

V

rails, maximizing the signal-to-noise ratio in a low voltage

system.

INSTRUMENTATION AMPLIFIER

Measurement of very small signals with an amplifier requires

close attention to the input impedance of the amplifier, the

overall signal gain from both inputs to the output, as well as,

the gain from each input to the output. This is because we

are only interested in the difference of the two inputs and the

common signal is considered noise. A classic solution is an

instrumentation amplifier. Instrumentation amplifiers have a

finite, accurate, and stable gain. Also they have extremely

high input impedances and very low output impedances.

Finally they have an extremely high CMRR so that the

amplifier can only respond to the differential signal.

Three-Op-Amp Instrumentation Amplifier

A typical instrumentation amplifier is shown in Figure 5.

There are two stages in this configuration. The last stage,

the output stage, is a differential amplifier. In an ideal case

the two amplifiers of the first stage, the input stage, would be

set up as buffers to isolate the inputs. However they cannot

be connected as followers due to the mismatch of real

amplifiers. The circuit in Figure 5 utilizes a balancing resistor

between the two amplifiers to compensate for this mismatch.

The product of the two stages of gain will be the gain of the

instrumentation amplifier circuit. Ideally, the CMRR should

be infinite. However the output stage has a small non-zero

common mode gain which results from resistor mismatch.

In the input stage of the circuit, current is the same across all

resistors. This is due to the high input impedance and low

input bias current of the LMV716. With the node equations

we have:

(1)

By Ohm’s Law:

(2)

However:

(3)

So we have:

(4)

20179515

FIGURE 4. Single-supply Inverting Amplifier

20179542

FIGURE 5. Three-Op-Amp Instrumentation Amplifier

www.national.com

10