A three-op-amp instrumentation amplifier designed and simulated in LTspice, built using LT1007 precision operational amplifiers. This circuit is designed for precision differential signal acquisition — commonly used for amplifying small differential signals from sensors while rejecting common-mode noise.
An instrumentation amplifier (in-amp) is used when you need to amplify a small voltage difference between two signal lines while ignoring any voltage that's common to both — for example, reading a sensor over long wires where both lines pick up the same electrical noise. This project implements the classic three-op-amp topology, which offers high input impedance, well-defined gain, and good common-mode rejection.
The design consists of two stages:
Input stage (U2, U3): Two op-amps (LT1007) configured as buffers, cross-coupled through a shared gain-setting resistor network. This stage amplifies the differential signal while keeping input impedance high — important since it avoids loading down whatever sensor or source is feeding the amplifier.
- R1 = 10 kΩ
- R2 = 20 kΩ (gain-setting resistor)
- R3 = 10 kΩ
Output stage (U1): A unity-gain difference amplifier that subtracts the two buffered signals, converting the differential signal into a single-ended output referenced to ground, while further rejecting any common-mode component that survived the first stage.
- R4 = R5 = R6 = R7 = 10 kΩ
First-stage gain (set by R1, R2, R3):
G1 = 1 + (R1 + R3) / R2 = 1 + (10k + 10k) / 20k = 2
Second-stage gain (difference amplifier, since R4 = R5 = R6 = R7):
G2 = R5 / R4 = 10k / 10k = 1
Total differential gain:
G_total = G1 × G2 = 2 × 1 = 2 (equivalent to 6.02 dB)
To verify the gain calculation, a DC differential input was applied: V1 = 10V, V2 = 0V (differential input = 10V).
Expected output: Vout = G_total × Vin,diff = 2 × 10V = 20V
Simulated output: Vout = −20V — the magnitude matches the hand-calculated gain exactly; the negative sign is simply due to which input (V1 vs V2) lands on the inverting side of the final difference amplifier (U1).
To characterize the amplifier's bandwidth, an AC sweep was run from 1 Hz to 10 MHz using the directive .ac dec 100 1 10Meg.
The resulting Bode plot was used to extract the closed-loop bandwidth and gain-bandwidth product directly from the simulation data.
Results:
| Parameter | Value |
|---|---|
| Passband gain | 2.0 V/V (6.02 dB) |
| Peak gain | 8.64 dB at 1.70 MHz |
| −3 dB bandwidth | 3.54 MHz |
| Gain-bandwidth product | 7.07 MHz |
The frequency response shows approximately 2.6 dB of peaking near 1.7 MHz before rolling off, rather than a smooth monotonic decline. This indicates reduced phase margin, caused by cascading two op-amp gain stages — each stage contributes a pole, and when two poles occur close together in frequency, the loop can show gain peaking near the point where phase margin is lowest.
| File | Description |
|---|---|
Instrumentation_Amplifier.asc |
LTspice schematic file |
Instrumentation_Amplifier.net |
Generated SPICE netlist |
images/schematic.png |
Screenshot of the circuit schematic |
images/transient_output.png |
Transient simulation output waveform |
images/ac_response.png |
AC sweep Bode plot (magnitude & phase) |
- LTspice 24.1.9
- Op-amp model: LT1007 (Analog Devices/Linear Technology)
Subrat Panda LinkedIn | GitHub | 24je0177@iitism.ac.in