An open loop op amp operates without a feedback network connecting the output to the inverting input. Because the open-loop voltage gain ($A_{OL}$) of a typical operational amplifier exceeds 100,000 (100 dB), even a millivolt difference between the inputs drives the output into saturation. In practice, you never use an open loop op amp for linear amplification; you use it as a comparator to make binary decisions based on voltage thresholds.
While it is tempting to grab a general-purpose op-amp like the LM358 and run it open-loop, doing so often leads to slow recovery times, output phase reversal, and destroyed input stages. This guide covers the exact specifications, safe default part numbers, and practical circuit designs required to implement reliable open-loop comparator circuits on the bench.
Pinout, Symbol, and Open Loop Operating Regions
The schematic symbol for an open loop op amp is identical to a standard op-amp—a triangle with a non-inverting input (+), an inverting input (-), positive and negative supply rails ($V_{CC}$ and $V_{EE}$), and an output. The defining visual difference is the absence of a feedback path routing from the output back to the inverting input.
For the industry-standard DIP-8 dual comparator package (like the LM393), the pinout is: Pin 1 (Output A), Pin 2 (Inverting Input A), Pin 3 (Non-Inverting Input A), Pin 4 (GND/$V_{EE}$), Pin 5 (Non-Inverting Input B), Pin 6 (Inverting Input B), Pin 7 (Output B), and Pin 8 ($V_{CC}$). Always verify the datasheet, as single comparators like the LM311 use a different pinout featuring offset null and strobe pins.
Operation Regions and Typical Specifications
Without negative feedback to stabilize the gain, the device operates in one of three distinct regions. The table below outlines the electrical behavior in each state based on a standard 12V single-supply configuration.
| Operating Region | Input Condition | Output Voltage (Typical) | Output Current Limit | Practical Application |
|---|---|---|---|---|
| Positive Saturation | $V_{IN+} > V_{IN-}$ | $V_{CC} - V_{drop}$ (approx 10.5V) | Source: Limited by pull-up | Logic HIGH, Relay ON |
| Negative Saturation | $V_{IN+} < V_{IN-}$ | $V_{EE}$ (approx 0.2V for BJTs) | Sink: 16mA to 20mA max | Logic LOW, Relay OFF |
| Linear (Theoretical) | $V_{IN+} \approx V_{IN-}$ | Unstable / Oscillating | N/A | Avoid (causes chatter) |
| Overdrive Recovery | Inputs cross threshold | Slew-rate limited transition | Transient spikes possible | Determines max switching freq |
Safe Default Part Numbers and Selection Criteria
A common beginner mistake is using a general-purpose operational amplifier (like the LM741 or LM358) as an open loop comparator. While they will technically work in a pinch, op-amps are internally compensated for stability in closed-loop configurations. When driven into saturation open-loop, internal transistor stages saturate and store charge. Recovering from this saturation takes microseconds— an eternity in high-speed digital logic. Furthermore, many older op-amps suffer from phase reversal if the input voltage exceeds the common-mode range, causing the output to flip to the wrong rail.
For dedicated open-loop applications, always select a true comparator IC. Comparators feature fast overdrive recovery, open-collector or open-drain outputs for easy logic-level translation, and are designed to handle large differential input voltages without damage.
Comparator IC Comparison Matrix
| Part Number | Type | Supply Range | Output Stage | Response Time | Best Use Case |
|---|---|---|---|---|---|
| LM393 | Dual Comparator | 2V to 36V | Open-Collector BJT | 1.3 µs | General purpose, battery monitors, 12V systems |
| LM311 | Single Comparator | 5V to 30V | Open-Collector / Emitter | 200 ns | Driving relays directly, isolated outputs |
| TLV3691 | Single Nano-Power | 1.6V to 5.5V | Push-Pull CMOS | 18 µs | Battery-powered ESP32/Arduino wake-up triggers |
| LM358 (Avoid) | Dual Op-Amp | 3V to 32V | Push-Pull BJT | ~10 µs (Recovery) | Linear amplification only (Not recommended open-loop) |
For 90% of hobbyist and DIY jobsite projects running on 5V to 12V, the LM393 is the safest default. It costs pennies, is available from every major distributor, and its open-collector output means you can pull the output up to a 3.3V logic rail even if the LM393 is powered by 12V, making it perfect for interfacing with ESP32 or Raspberry Pi GPIO pins.
Practical Application: 12V Low-Voltage Cutoff Circuit
Below is a complete, tested application circuit using an LM393 configured as an open loop op amp to monitor a 12V lead-acid battery. When the battery drops below 11.8V, the output sinks current to ground, illuminating an LED and signaling a microcontroller.
Component List and Values
- U1: LM393 Dual Comparator (using Channel A)
- R1: 10 kΩ (Voltage divider top)
- R2: 3.3 kΩ (Voltage divider bottom, sets reference)
- R3: 1 MΩ (Hysteresis feedback resistor)
- R4: 470 Ω (LED current limiting)
- D1: Standard 5mm Red LED
- R5: 10 kΩ (Pull-up resistor to 5V logic)
Wiring Steps
- Establish the Reference: Connect R1 between the 12V battery positive and Pin 3 (Non-Inverting Input). Connect R2 between Pin 3 and Ground. This creates a stable reference voltage of roughly 2.97V at Pin 3.
- Route the Sense Voltage: Feed the battery voltage through a separate divider (or directly if scaled) to Pin 2 (Inverting Input). For this example, assume Pin 2 sees 3.0V when the battery is at 12.0V, and drops to 2.95V when the battery hits 11.8V.
- Add Hysteresis: Connect R3 (1 MΩ) between Pin 1 (Output) and Pin 3 (Non-Inverting Input). This prevents output chatter when the battery voltage hovers exactly at the threshold.
- Configure the Output: Connect R5 (10 kΩ) between Pin 1 and your 5V microcontroller logic rail. Connect R4 and D1 in series from Pin 1 to Ground.
- Power the IC: Connect Pin 8 to the 12V battery positive and Pin 4 to Ground.
When the battery is healthy (>11.8V), the voltage at Pin 2 is higher than Pin 3. The open-collector output turns on, pulling Pin 1 to ground. The LED remains off, and the microcontroller reads 0V (LOW). When the battery sags below 11.8V, Pin 2 drops below Pin 3. The output transistor turns off, allowing R5 to pull Pin 1 to 5V. The LED illuminates, and the microcontroller reads 5V (HIGH).
Biasing, Hysteresis, and Preventing Oscillation
The most frequent point of failure in open loop op amp designs is high-frequency oscillation, commonly called 'chatter'. Real-world signals are noisy. If a 60Hz AC ripple or switching noise from a buck converter is superimposed on your DC input, the inputs will rapidly cross back and forth over the threshold. Because the open-loop gain is 100,000, a 10µV noise spike is amplified to 1V, causing the output to violently toggle between rails.
To solve this, you must bias the circuit with hysteresis using positive feedback. By routing a high-value resistor (like the 1 MΩ R3 in the circuit above) from the output back to the non-inverting input, you create two distinct thresholds: one for turning on, and a slightly lower one for turning off. This dead-band ensures that once the output flips, the input noise must exceed the hysteresis window to flip it back. According to Analog Devices Tutorial MT-083, calculating the exact hysteresis window requires matching the feedback resistor to the source impedance of your voltage divider, but a 1 MΩ to 10 MΩ resistor is a safe starting point for low-frequency DC monitoring.
Failure Modes and Multimeter Diagnostics
Comparators are rugged, but they do fail, usually due to input overvoltage or output short circuits. Here is how to diagnose a suspected dead IC using a standard digital multimeter (DMM).
How Open Loop Op Amps Fail
- Input Differential Overvoltage: The TI LM393 Datasheet specifies a maximum differential input voltage of ±36V. Exceeding this breaks down the base-emitter junctions of the input PNP transistors, permanently altering the input offset voltage or shorting the inputs.
- Output Short to VCC: Because the output is open-collector, shorting the output pin directly to $V_{CC}$ while the internal transistor is pulling to ground will instantly destroy the output silicon due to thermal runaway.
- Latch-up: If the input voltage exceeds the supply voltage ($V_{CC}$), parasitic SCRs inside the silicon can trigger, shorting $V_{CC}$ to ground and destroying the chip.
Multimeter Testing Procedure
To verify an LM393 or similar comparator on the bench, remove it from the circuit (or ensure power is completely disconnected) and set your DMM to Diode Test mode.
- Test the Output Transistor: Place the red probe on the Output pin (Pin 1 or 7) and the black probe on Ground (Pin 4). You should read an open circuit (OL). Reverse the probes; you should read a standard silicon diode drop (approx 0.6V to 0.7V). If it reads 0.00V or shorted in both directions, the output transistor is blown.
- Test the Input Stage: Measure between the Inverting and Non-Inverting inputs. You should read a high resistance or diode drop depending on the internal protection diodes. If it reads a dead short, the input stage has suffered overvoltage breakdown.
- Live Voltage Verify: If the IC is powered, switch your DMM to DC Voltage. Measure the reference pin (e.g., Pin 3). Then measure the output pin. If the output pin is stuck at exactly $V_{CC}$ minus a diode drop regardless of input conditions, the internal current source or pull-up network has failed.
