When you route a portion of an amplifier's output back to its non-inverting input, you create a positive feedback op amp configuration. Unlike negative feedback, which forces the device into a linear, predictable amplification state, positive feedback aggressively reinforces any differential voltage between the inputs. This snaps the output instantly to the positive or negative supply rail. In practical circuit design, we use this behavior to build Schmitt triggers (comparators with hysteresis for noise immunity), astable multivibrators (square wave oscillators), and latching relays. If you need a general-purpose, low-frequency default part for this topology, buy the LM393 dual comparator. If you need high-speed, rail-to-rail performance, use the TLV3201. Below is the bench-tested guide to designing, biasing, and troubleshooting positive feedback networks without falling into the common traps that brick prototypes.

Symbol, Pinout, and the Positive Feedback Mechanism

The standard schematic symbol for an op-amp or comparator is a triangle pointing right. The non-inverting input is marked with a plus (+), the inverting input with a minus (-), and the output emerges from the apex. In a positive feedback configuration, the critical path connects the output pin back to the non-inverting (+) input.

Standard 8-Pin DIP Pinout

Whether you are holding an LM358 op-amp or an LM393 comparator in a through-hole DIP-8 package, the industry-standard pinout is:

  • Pin 1: Output A
  • Pin 2: Inverting Input A (-)
  • Pin 3: Non-Inverting Input A (+)
  • Pin 4: V- (Ground or negative rail)
  • Pin 5: Non-Inverting Input B (+)
  • Pin 6: Inverting Input B (-)
  • Pin 7: Output B
  • Pin 8: V+ (Positive supply rail)
Bench Warning: Never use a general-purpose op-amp (like the LM358 or TL072) for high-speed positive feedback switching. Op-amps are designed for linear operation; when driven into saturation by positive feedback, they suffer from slow slew rates and "phase reversal" on the inputs. Always use a dedicated comparator IC for Schmitt triggers.

Operation Regions and Hysteresis Thresholds

A positive feedback op amp operates strictly in the saturation regions, intentionally avoiding the linear region. The transition between these states is governed by hysteresis—the intentional gap between the upper and lower switching thresholds that prevents output chatter when the input signal is noisy.

Operation Region Input State (V+ vs V-) Output Voltage (5V Vcc) Feedback Effect
Positive Saturation (High) V+ > V- (by > 1mV) ~4.8V (Push-Pull) or Floating (Open-Collector) Reinforces V+, locking output HIGH
Negative Saturation (Low) V+ < V- (by > 1mV) ~0.1V to 0.2V (Saturation voltage) Reinforces V-, locking output LOW
Transition (Hysteresis Gap) V+ ≈ V- (within threshold window) Rapidly slewing between rails Positive feedback accelerates the flip

Designing an Inverting Schmitt Trigger Circuit

The most common positive feedback application is the Inverting Schmitt Trigger. The input signal goes to the inverting (-) pin, while the non-inverting (+) pin receives a voltage divider tied between a reference voltage and the output pin.

Component Values and Schematic

  • IC: LM393 (Dual Comparator, Open-Collector Output)
  • Vcc: 5.0V DC
  • R_pullup: 10kΩ (from Output to 5V Vcc) — Required because LM393 is open-collector.
  • R1 (Feedback): 100kΩ (from Output to Non-Inverting Pin 3)
  • R2 (Reference): 10kΩ (from 2.5V Reference to Non-Inverting Pin 3)
  • Vref: 2.5V (Derived from a separate 10k/10k divider off the 5V rail)

Calculating the Hysteresis Window

Because the LM393 output pulls to ground (0V) when LOW, and floats (allowing the 10k pull-up to pull it to 5V) when HIGH, we calculate the Upper Trigger Point (UTP) and Lower Trigger Point (LTP) at the non-inverting pin:

When Output is HIGH (5V):
The voltage at Pin 3 is the parallel combination of R1 and R2.
V_UTP = (5V × 10k + 2.5V × 100k) / (100k + 10k) = 2.72V.
The input signal must rise above 2.72V to force the output LOW.

When Output is LOW (0V):
V_LTP = (0V × 10k + 2.5V × 100k) / (100k + 10k) = 2.27V.
The input signal must fall below 2.27V to force the output HIGH.

Total Hysteresis: 2.72V - 2.27V = 0.45V. This 450mV deadband ensures that 60Hz mains hum or switch bounce on the input signal will not cause the output to chatter.

Decision Path: Selecting the Right Comparator or Op-Amp

Choosing the wrong IC for a positive feedback circuit leads to slow edges, missed pulses, or destroyed output stages. Use this decision matrix to terminate your part selection.

If your application requires... Then choose this architecture... Concrete Part Number
< 100kHz switching, 5V-12V single supply, lowest cost Open-Collector Comparator LM393 (Add 10k pull-up)
> 1MHz switching, clean CMOS logic levels, no pull-up Push-Pull Rail-to-Rail Comparator TLV3201 or MAX902
Battery powered (< 5µA quiescent), ultra-low speed Nano-Power Comparator TLV3691
You MUST use an existing op-amp inventory (not recommended) Modern Rail-to-Rail Op-Amp (No phase reversal) TLV9001 (Avoid LM358/LM324)
Pro Tip: If you are forced to use an op-amp like the TLV9001 as a comparator, clamp the differential input voltage with two 1N4148 diodes (anode to cathode across the inputs). Op-amp input stages can be damaged by large differential voltages during the slow saturation transition; comparators are built to tolerate them.

Failure Modes and Multimeter Diagnostics

Positive feedback circuits fail in predictable ways. The most common bench failure is output stage destruction caused by shorting an open-collector output directly to Vcc without a pull-up resistor, or exceeding the common-mode input voltage range, which causes internal latch-up.

How to Test with a Digital Multimeter (DMM)

Do not rely on resistance checks alone. Follow this powered and unpowered diagnostic sequence:

  1. Unpowered Input Diode Test: Set your DMM to diode mode. Place the red probe on the Inverting (-) pin and black on V-. Then swap to the Non-Inverting (+) pin. You should read a forward voltage drop between 0.6V and 0.8V (the internal ESD protection diodes). A reading of 0.0V indicates a shorted input stage; an "OL" reading indicates a blown internal bond wire.
  2. Unpowered Output Short Check: In resistance mode, measure between the Output pin and Vcc, and Output to GND. You should see high resistance (MΩ range). A low resistance (< 50Ω) means the output transistor is shorted.
  3. Powered Rail Verification: Power the circuit. Measure Pin 8 (V+) to Pin 4 (GND). It must read exactly your supply voltage (e.g., 5.00V). If it reads 2V, the IC is in thermal shutdown or latch-up, drawing massive current.
  4. The "Force-Flip" Test: Power the circuit. Use a 10k potentiometer to inject a variable voltage into the inverting (-) input. Slowly sweep the voltage from 0V to 5V. The output should snap cleanly from HIGH to LOW at the UTP (2.72V in our example circuit). Sweep back down; it must snap from LOW to HIGH at the LTP (2.27V). If the output lingers in a linear state (e.g., 2.5V) or oscillates wildly, your positive feedback resistor (R1) is open or the wrong value.

The Bench-Tested Safe Defaults for 2026

When stocking your lab or finalizing a production BOM, these two part numbers cover 95% of positive feedback op amp and comparator use cases.

1. The General Purpose Workhorse: Texas Instruments LM393

The LM393 remains the undisputed king of low-frequency, cost-sensitive comparator designs. It features an open-collector output, meaning you can pull the output up to a different voltage than the IC's Vcc (e.g., run the LM393 at 5V, but pull the output up to 12V to drive a relay directly).
Key Ratings: Vcc: 2V to 36V (single supply). Quiescent current: 0.4mA per channel. Propagation delay: 1.3µs.
Cost: ~$0.15 per unit in DIP-8.

2. The High-Speed Modern Default: Texas Instruments TLV3201

If you are digitizing high-frequency sensor signals or building a fast PWM modulator, the LM393's 1.3µs delay will ruin your timing. The TLV3201 is a push-pull, rail-to-rail comparator that interfaces directly with 3.3V and 5V microcontrollers without pull-up resistors.
Key Ratings: Vcc: 2.7V to 5.5V. Propagation delay: 40ns. Output drive: ±50mA.
Cost: ~$0.85 per unit in SOT-23-5.

By strictly applying positive feedback to the non-inverting node, calculating your hysteresis deadband before soldering, and selecting a dedicated comparator rather than a linear op-amp, you will eliminate output chatter and ensure rock-solid digital transitions on the bench.