Non-Inverting Amplifier Circuit Diagram: Av = 1+Rf/Rin, High Impedance & Design
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A non-inverting amplifier is an op-amp configuration where the input signal is applied directly to the non-inverting (+) terminal, feedback is provided via a resistor divider (Rf and Rin) from the output back to the inverting (−) terminal, and the gain is Av = 1 + Rf/Rin — always positive, so the output is in-phase with the input. The high input impedance makes it ideal for buffering sensor outputs and impedance transformation. When Rf = 0 (or Rin = ∞), it becomes a unity-gain voltage follower.
The non-inverting amplifier is the second fundamental op-amp topology, complementing the inverting amplifier. Its key advantages are very high input impedance (the signal drives the high-impedance (+) input directly), positive phase (in-phase output), and gain always ≥ 1.
**Circuit Topology**
- Input: Vin applied to non-inverting (+) terminal - Inverting (−) terminal connected to the junction of Rf and Rin - Rf connected between output and inverting input (feedback resistor) - Rin connected between inverting input and ground - Output: Vout at op-amp output pin
**Gain Derivation**
Using the virtual short-circuit principle (V+ = V- due to high gain + feedback): - V+ = Vin (since input directly drives the + terminal) - V- = Vout × Rin / (Rin + Rf) (voltage divider from output to ground) - Since V+ = V-: Vin = Vout × Rin / (Rin + Rf) - Solving: Av = Vout/Vin = 1 + Rf/Rin
The gain is always ≥ 1 (unlike the inverting amplifier where gain can be < 1 in magnitude). The output is in-phase (positive gain, no inversion).
**Voltage Follower (Unity-Gain Buffer)**
When Rf = 0 (short circuit) and Rin = ∞ (open circuit or removed), the gain equation becomes Av = 1 + 0 = 1. This is the voltage follower or unity-gain buffer: - Vout = Vin exactly - Input impedance: very high (op-amp intrinsic, typically 1 MΩ to 10^12 Ω for JFET inputs) - Output impedance: nearly 0Ω - Used to isolate stages and drive low-impedance loads from high-impedance sources
**Input Impedance**
The input impedance of the non-inverting amplifier is approximately the op-amp's intrinsic differential input impedance multiplied by the loop gain — effectively tens or hundreds of MΩ in practice. This is far higher than the inverting amplifier's Zin = Rin, making the non-inverting topology essential for high-impedance sensor inputs (pH electrodes, piezo sensors, thermocouples).
**Output Impedance**
With negative feedback, output impedance ≈ Rout_open_loop / (1 + Aol × β) ≈ near 0Ω, where β = Rin/(Rin+Rf) is the feedback factor.
**Bandwidth**
As with the inverting amplifier, BW = GBW / Av = GBW × Rin / (Rin + Rf). For Av=2 with a TL071 (GBW=3MHz): BW = 1.5 MHz.
**Comparison: Inverting vs Non-Inverting**
| Property | Inverting | Non-Inverting | |---|---|---| | Gain formula | Av = -Rf/Rin | Av = 1+Rf/Rin | | Phase | 180° (inverted) | 0° (in-phase) | | Min gain magnitude | Any (incl. <1) | 1 (always ≥1) | | Input impedance | Rin | Very high (MΩ+) | | Virtual ground | Yes, at V- | No |
**Common Op-Amp Choices**
- LM741: Classic, ±15V, GBW 1 MHz, 8-pin DIP (pinout: V+=3, V-=2, Out=6, Vcc+=7, Vcc-=4) - TL071/TL081: JFET input, very high input impedance, low bias current (50 pA vs 80 nA for 741) - LM358: Single or dual supply, works down to 3V - AD8605: Rail-to-rail CMOS, very low offset, single supply
**Gain Setting Example**
For Av = +11: choose Rin = 1 kΩ, then Rf = (Av-1) × Rin = 10 × 1kΩ = 10 kΩ.
For Av = +2 (common buffer with gain): Rin = Rf (any equal pair, e.g. both 10 kΩ).
Build your non-inverting amplifier circuit diagram in the free circuitdiagrammaker.com editor. Connect the input to the (+) terminal, wire Rin from (−) to ground, and Rf from output to (−). Adjust values and observe in-phase gain with no output inversion.
How to wire non inverting amplifier circuit diagram
- Select the op-amp For general purposes use LM741 or LM358. For high-impedance sensor inputs, use TL071 or TL081 (JFET input, 50 pA bias current vs 80 nA for bipolar op-amps). For single-supply applications, use LM358 or rail-to-rail types.
- Calculate resistor values Determine required gain Av. Then: Rf = (Av - 1) × Rin. Choose Rin first (e.g. 10 kΩ), then Rf = (Av-1) × Rin. For Av=6: Rf = 5 × 10 kΩ = 50 kΩ. Use nearest standard value (47 kΩ for a slight gain trim).
- Connect the op-amp supply For LM741/TL071 with ±15V supplies: connect pin 7 to +15V, pin 4 to -15V. Place 100nF ceramic capacitors from each supply pin to ground, positioned close to the IC.
- Wire the input to the non-inverting terminal Connect the input signal source directly to the non-inverting (+) pin (pin 3 on LM741). No resistor is needed in series for basic operation, though a small series resistor (100Ω) can protect against ESD.
- Wire Rin and Rf for feedback Connect Rin between the inverting (−) input (pin 2 on LM741) and ground. Connect Rf between the op-amp output (pin 6) and pin 2 (inverting input). This closes the negative feedback loop.
- For a voltage follower Remove Rin and Rf entirely. Connect a short circuit (wire) directly from pin 6 (output) to pin 2 (inverting input). The output follows the input with unity gain and very high input impedance.
- Verify gain and phase Apply a sine wave (e.g. 100mV, 1kHz) to the input. Measure output with oscilloscope. Confirm Vout ≈ Av × Vin, and that the waveforms are in phase (no inversion). Check at multiple frequencies to confirm bandwidth is adequate.
Specifications
| Voltage gain formula | Av = 1 + Rf/Rin |
|---|---|
| Minimum possible gain | Av = 1 (when Rf=0 or Rin=∞) |
| Phase shift | 0° (output in phase with input) |
| Input impedance | Very high (≈ Aol × β × Rin_opamp, typically >10MΩ) |
| Output impedance (ideal) | ≈ 0 Ω |
| Voltage follower gain | Av = 1 (Rf=0, Rin removed) |
| Bandwidth | BW = GBW / Av |
| LM741 GBW | 1 MHz |
| TL071 GBW | 3 MHz (JFET input, 50 pA bias current) |
| Gain of +11 example | Rin=1kΩ, Rf=10kΩ |
| Gain of +2 example | Rin=Rf (e.g. both 10kΩ) |
| Feedback factor β | β = Rin / (Rin + Rf) |
Safety warnings
- Do not exceed the op-amp's maximum supply voltage — the LM741 and TL071 have an absolute maximum of ±18V; exceeding this destroys the IC immediately.
- When powering the op-amp from a dual supply (e.g. ±15V), connect the negative supply pin before applying the positive supply to avoid momentary single-supply latch-up during power sequencing in sensitive circuits.
Tools needed
- Op-amp IC: LM741, TL071, LM358, or equivalent
- Resistors: Rin and Rf (1kΩ to 100kΩ, 1% metal film for precision gain)
- 100nF ceramic decoupling capacitors (one per supply rail)
- Dual-rail power supply (±15V for LM741) or single supply (3–32V for LM358)
- Signal generator (for sine wave input) and oscilloscope (for output verification)
- Multimeter for DC bias and supply voltage checks
Common mistakes
- Grounding the non-inverting input (+) instead of connecting the signal there: this turns the circuit into an inverting configuration. The signal must go to the (+) pin for a non-inverting amplifier.
- Connecting Rf from output to the non-inverting (+) input instead of the inverting (−) input: this creates positive feedback, causing the op-amp to saturate. Rf must always go to the inverting (−) terminal.
- Forgetting to include Rin to ground from the inverting input: without Rin, the feedback divider has no reference to ground and the gain is undefined (approaches open-loop gain — effectively just a comparator).
- Using the LM741 at gains close to 1 for high-frequency signals: the LM741's 1 MHz GBW means at Av=1, bandwidth is 1 MHz but slew rate is only 0.5V/µs, which severely limits large-signal high-frequency performance. Use a faster op-amp for such applications.
Troubleshooting
- Output saturated at supply rail with no input applied
- Cause: Rf connected to non-inverting (+) input (positive feedback) instead of inverting (-) Fix: Remove power. Trace Rf — it must go from output to the (-) pin (pin 2 on LM741). Move the wire to the correct pin.
- Gain is correct but output is inverted (180° phase shift)
- Cause: Input signal accidentally connected to the inverting (-) input instead of non-inverting (+) Fix: Confirm input wire goes to pin 3 (non-inverting, +) on LM741, not pin 2 (inverting, -). Swap the connection.
- Large DC offset at output with zero AC input
- Cause: Op-amp bias current flowing through Rf creating an offset, or Rin missing Fix: Ensure Rin is connected from (-) to ground. For LM741, the input bias current (~80nA) × Rf (100kΩ) = 8mV offset is expected. Use a TL071 (50pA bias) for lower offset, or add offset null potentiometer (LM741 pins 1 and 5).
- High-frequency output looks distorted or clipped
- Cause: Slew rate limiting: input signal amplitude × frequency exceeds op-amp slew rate Fix: Check op-amp slew rate (LM741: 0.5 V/µs). Maximum non-distorted sine amplitude at frequency f: Vpeak ≤ SR/(2πf). Use a faster op-amp or reduce signal amplitude.
Frequently asked questions
What is the gain of a non-inverting amplifier?
Av = 1 + Rf/Rin. The gain is always ≥ 1 and the output is in-phase with the input (no inversion). For Rf=10kΩ and Rin=1kΩ, Av = 1 + 10 = 11.
What is a voltage follower and how is it related to the non-inverting amplifier?
A voltage follower is a non-inverting amplifier with Av=1, achieved by connecting the output directly to the inverting input (Rf=0, Rin removed). It provides very high input impedance and very low output impedance for impedance buffering.
Why does a non-inverting amplifier have high input impedance?
The input signal drives the non-inverting (+) terminal of the op-amp directly. The (+) input has the op-amp's intrinsic input impedance (1MΩ to >100GΩ for JFET), multiplied further by the negative feedback loop gain. This makes it ideal for high-impedance sources.
What is the minimum gain of a non-inverting amplifier?
The minimum gain is Av=1 (unity), achieved when Rf=0 or Rin=∞. Unlike the inverting amplifier, the non-inverting configuration cannot achieve gain less than 1.
What is the difference between an inverting and non-inverting amplifier?
The inverting amplifier (Av = -Rf/Rin) inverts the output (180° phase shift) and has input impedance = Rin. The non-inverting amplifier (Av = 1+Rf/Rin) is in-phase, has very high input impedance, and minimum gain of 1.
Which op-amp should I use for a non-inverting amplifier with a sensor input?
For high-impedance sensor inputs (pH electrodes, piezo), use a JFET-input op-amp like TL071 or TL081 (input bias current ~50 pA) or an instrumentation amplifier like INA128. Bipolar op-amps like LM741 (80 nA bias) can cause significant offset with high source impedances.
How do I set the bandwidth of a non-inverting amplifier?
Bandwidth = GBW / Av. For a required bandwidth of 100 kHz at gain 5, you need GBW ≥ 500 kHz. An LM741 (GBW=1MHz) would give BW=200kHz at Av=5, which is adequate. For wider bandwidth, choose a faster op-amp.