When you wire an operational amplifier with negative feedback, it behaves as a predictable, linear amplifier. But when you route a portion of the output signal back to the non-inverting (+) input, you introduce op amp positive feedback. This intentionally destabilizes the linear region, forcing the output to slam into the supply rails. While this ruins linear amplification, it is exactly the mechanism required to build comparators with hysteresis (Schmitt triggers), astable multivibrators, and latch circuits.
This guide covers the practical bench realities of positive feedback: how to select the right IC, how to wire a reliable Schmitt trigger, and how to test these devices when they fail on your workbench.
The Core Mechanism: Driving the Op-Amp to the Rails
In a standard linear amplifier, negative feedback corrects output errors, maintaining the virtual short between the inverting and non-inverting inputs. Positive feedback does the opposite: it reinforces the error. If the non-inverting input voltage rises even a microvolt above the inverting input, the output swings positive. That positive swing feeds back to the non-inverting input, driving it even higher, until the output hits the positive supply rail (saturation).
Standard 8-Pin DIP Pinout (LM358 / LM393)
To implement this on the bench, you need to know your pinout. The industry-standard dual op-amp (LM358) and dual comparator (LM393) share the same 8-pin DIP footprint:
- Pin 1: Output A
- Pin 2: Inverting Input A (-)
- Pin 3: Non-Inverting Input A (+)
- Pin 4: GND / V- (Negative Supply)
- Pin 5: Non-Inverting Input B (+)
- Pin 6: Inverting Input B (-)
- Pin 7: Output B
- Pin 8: VCC / V+ (Positive Supply)
Safe Default Part Numbers and Selection Criteria
Not all op-amps handle positive feedback gracefully. Standard op-amps can suffer from slow slew rates or phase inversion when overdriven. For positive feedback applications, you are usually building a comparator. Here are the safe default part numbers, complete with their critical ratings.
| Part Number | Type | Supply Range | Output Stage | Typical Cost (2026) | Best Application |
|---|---|---|---|---|---|
| LM358 | Standard Op-Amp | 3V to 32V | Push-Pull (Losses ~1.5V at top rail) | $0.12 | Low-speed, single-supply Schmitt triggers |
| LM393 | Dedicated Comparator | 2V to 36V | Open-Collector (Requires pull-up resistor) | $0.15 | Robust industrial switching, level translation |
| TLV3691 | Modern Rail-to-Rail Comparator | 1.8V to 5.5V | Push-Pull (True rail-to-rail) | $1.45 | Battery-powered devices, 3.3V logic interfacing |
Operation Regions and Typical Electrical Behavior
Understanding how the IC behaves across different feedback topologies is critical for debugging. The table below maps the operation regions using the LM358 on a 5V single supply.
| Region | Feedback Type | Output State | Typical Vout | Typical Iout (Source/Sink) |
|---|---|---|---|---|
| Linear | Negative | Proportional to input | 0.1V to 3.5V | ±20 mA max |
| Saturation (High) | Positive | Railed High | ~3.5V (VCC - 1.5V) | Source: 30 mA |
| Saturation (Low) | Positive | Railed Low | ~0.05V | Sink: 50 mA |
Notice the high-state saturation voltage. The LM358 cannot swing true rail-to-rail on the high side. If your positive feedback circuit requires a 5V logic high to trigger a microcontroller GPIO, the LM358 will fail you. You must use the TLV3691 or an LM393 with a pull-up resistor to the 5V rail. For more on op-amp output stage limitations, refer to the Texas Instruments Op-Amp Learning Hub.
Step-by-Step Build: Inverting Schmitt Trigger Circuit
The most common application of op amp positive feedback is the Schmitt trigger, which adds hysteresis to a comparator to prevent output chatter when dealing with noisy input signals. Below is a complete, bench-tested design for an Inverting Schmitt Trigger using the LM358.
Component List and Values
- U1: LM358 Dual Op-Amp (using channel A only)
- R1: 10 kΩ (Reference divider to GND)
- R2: 10 kΩ (Reference divider to VCC)
- R_in: 10 kΩ (Input signal to Inverting pin)
- R_f: 100 kΩ (Positive feedback resistor)
- Power: 5V DC single supply
Wiring Numbered Steps
- Establish the Reference Voltage: Connect R1 (10k) from Pin 3 (Non-Inverting) to GND (Pin 4). Connect R2 (10k) from Pin 3 to VCC (Pin 8). This creates a rigid 2.5V reference at Pin 3.
- Wire the Positive Feedback: Connect R_f (100k) from Pin 1 (Output A) to Pin 3 (Non-Inverting). This is the critical positive feedback path that creates the hysteresis window.
- Route the Input Signal: Connect your noisy input signal through R_in (10k) to Pin 2 (Inverting Input).
- Terminate Unused Channels: To prevent the unused channel B from oscillating and injecting noise, tie Pin 6 (Inverting B) to Pin 1 (Output A) or GND, and tie Pin 5 (Non-Inverting B) to GND. Never leave op-amp inputs floating.
How the Hysteresis Works
Because of R_f, the voltage at Pin 3 is not a fixed 2.5V; it shifts slightly depending on the output state. When the output is Low (~0.05V), R_f pulls Pin 3 down to roughly 2.27V. The input at Pin 2 must rise above 2.27V to force the output High. Once the output snaps High (~3.5V), R_f pulls Pin 3 up to roughly 2.73V. The input must now drop below 2.73V to force the output Low again. This 0.46V gap is your hysteresis, effectively filtering out input noise smaller than 460mV. For deeper mathematical modeling of these thresholds, the Analog Devices Op-Amp Basics guide provides excellent derivation formulas.
Bench Testing: How Op-Amps Fail and How to Test Them
Op-amps in positive feedback circuits usually fail due to output short circuits, exceeding the absolute maximum common-mode input voltage, or electrostatic discharge (ESD) on the input pins. Here is how to diagnose a suspected dead IC using a standard digital multimeter (DMM).
Step 1: Cold Diode Test (Power Off)
Remove the IC from the circuit or ensure power is completely disconnected. Set your DMM to the diode test mode.
- Place the red probe on Pin 4 (GND) and the black probe on Pin 2 (Inverting) and Pin 3 (Non-Inverting). You should read a forward voltage drop of roughly 0.6V to 0.8V (the internal ESD protection diodes).
- Reverse the probes. The meter should read 'OL' (Open Loop). If it reads near 0V or beeps continuously, the input protection diodes are shorted, and the IC is dead.
Step 2: Power-On Quiescent Check
Power the circuit with 5V. With no input signal applied (tie Pin 2 to GND), measure the voltage at Pin 1 (Output). Because Pin 2 (0V) is lower than Pin 3 (~2.5V), the output should be railed high. On an LM358, you should read between 3.3V and 3.6V. If you read 0V, the output transistor is blown or the IC lacks power.
Step 3: Force Saturation Test
Temporarily jumper Pin 2 to VCC (5V). Pin 2 is now higher than Pin 3. The positive feedback should instantly snap the output to the low rail. Measure Pin 1; it should read less than 0.1V. If the output remains high, or floats at an intermediate voltage like 1.8V, the internal output stage has failed, or the positive feedback resistor (R_f) is open.
Frequently Asked Questions
Why does my op-amp oscillate when I accidentally wire positive feedback?
If you wire positive feedback without a deliberate hysteresis network (like the R_f and R_in divider), or if you add a capacitive load to the output, the phase shift can turn your DC saturation circuit into an unintended AC oscillator. The output will rapidly toggle between the high and low rails. To fix this, ensure your feedback network is purely resistive for DC Schmitt triggers, and keep capacitive loads under 50pF unless you add a series isolation resistor (typically 10Ω to 50Ω) directly at the output pin.
Can I use an LM358 op-amp instead of a dedicated comparator like the LM393?
You can, but with caveats. The LM358 is internally compensated for negative feedback stability, which intentionally slows down its slew rate (typically 0.5V/µs). When used with positive feedback as a comparator, the LM358 will take several microseconds to transition between rails. If you are switching a relay at 10Hz, the LM358 is fine. If you are decoding a 100kHz PWM signal, the LM358 will output a distorted triangle wave instead of a square wave. For anything above 10kHz, use a dedicated comparator like the LM393 or TLV3691.
How do I calculate the exact hysteresis voltage thresholds for my circuit?
For the inverting Schmitt trigger detailed above, the thresholds depend on the reference voltage (V_ref) and the resistor ratio. The upper threshold (V_th_high) is calculated as: V_ref * (1 + R1/R2) - V_OL * (R1/R2). The lower threshold (V_th_low) is: V_ref * (1 + R1/R2) - V_OH * (R1/R2). The total hysteresis window is simply the difference between V_th_high and V_th_low. If you need a wider hysteresis window to reject heavy noise, decrease the value of R_f relative to R_in.
