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TL072 op-amp configured as an inverting amplifier with a gain of -10 V/V. Standard textbook topology with bias compensation and supply bypassing.
| REF | TYPE | VALUE | ROLE |
|---|---|---|---|
| U1 | Operational amplifier | TL072 | Active gain element — JFET-input op-amp providing the high open-loop gain that makes the feedback math work. |
| Rin | Resistor | 10 kΩ | Input resistor — converts the input voltage into a current flowing into the virtual ground summing node. |
| Rf | Resistor | 100 kΩ | Feedback resistor — sets the closed-loop gain as -Rf/Rin = -10. |
| Rb | Resistor | 9.1 kΩ | Bias-compensation resistor on the non-inverting input — matches the DC source impedance seen at the inverting input to cancel input-bias-current offset. |
| C1, C2 | Capacitor | 100 nF | Power-supply bypass capacitors on ±V rails — supply transient current to the output stage during fast slewing. |
5 COMPONENTS IDENTIFIED
STAGES · 4
Input current conversion
Rin converts Vin into a current flowing into the inverting summing node (which the op-amp holds at virtual ground).
→ Rin
Active gain stage
U1 senses any imbalance at its inverting input and drives the output to keep it at the same potential as the grounded non-inverting input.
→ U1
Feedback / current return
Rf carries the input current from the summing node to the output. Because the op-amp's input draws ~0 current, all of Iin must flow through Rf, forcing Vout = -Vin × (Rf/Rin).
→ Rf
Bias-current compensation
Rb matches the impedance at V+ to (Rin ∥ Rf), so equal input bias currents create equal voltage drops and cancel at the output.
→ Rb
FEEDBACK PATHS
Rf carries the output back to the inverting input — classic shunt-shunt negative feedback. The loop drives the summing node to a virtual ground.
KEY NODES
DOMAIN
audio
INDUSTRY
Pro audio, instrumentation, sensor signal conditioning, and lab equipment. The TL072 has been in continuous production since 1978 and is still the default jellybean op-amp for non-precision audio work.
FREQUENCY
DC to ~300 kHz (closed-loop, limited by TL072's 3 MHz GBW divided by closed-loop gain of 10)
IMPEDANCE
Input impedance = Rin = 10 kΩ. Output impedance < 1 Ω at audio frequencies thanks to feedback.
APPLICATION
Inverting gain block — appears in audio preamps, instrumentation chains, summing mixers, current-to-voltage converters (photodiode amplifiers), and as the active element inside virtually every active filter topology.
OPERATING PRINCIPLE
The op-amp's enormous open-loop gain (~200,000) combined with negative feedback through Rf forces the inverting input to track the grounded non-inverting input — creating a 'virtual ground'. Any input voltage at Vin produces a current Iin = Vin/Rin into the summing node. Because the op-amp's input current is negligible, all of that current must flow through Rf to the output, dropping a voltage of Iin × Rf across it. Since the summing node is at 0 V, Vout = -Iin × Rf = -Vin × (Rf/Rin). The output is inverted because positive Iin must come from the output side of Rf, requiring Vout to swing negative.
KEY PARAMETERS
Closed-loop gain
-10V/V
-Rf/Rin = -100kΩ/10kΩ
Input impedance
10kΩ
Equal to Rin — independent of frequency
Bandwidth (-3dB)
~300kHz
GBW / |gain| = 3 MHz / 10
Slew rate
13V/µs
Limits large-signal bandwidth above ~200 kHz at 10 Vpp
Output noise (10 kHz)
180nV/√Hz
Dominated by TL072's 18 nV/√Hz input noise × gain
DC offset (typical)
±3mV
Vio × (1 + Rf/Rin), trimmable
DESIGN DECISIONS
Choosing Rf = 100 kΩ and Rin = 10 kΩ is a deliberate balance. Going larger increases noise (Johnson noise of the resistors grows with √R) and makes the circuit more sensitive to PCB leakage and stray capacitance. Going smaller wastes drive current — at high signal levels the op-amp output stage has to supply Vout/Rf into the feedback resistor. Ten-to-one is a popular gain because it gives ample headroom (TL072 GBW of 3 MHz / gain of 10 = 300 kHz, comfortably above the 20 kHz audio band) while still being useful gain. The 9.1 kΩ bias-compensation resistor equals 10k ∥ 100k = 9.09k — picking 9.1k (E96 series) keeps mismatch under 0.2%. The TL072 specifically is chosen for JFET inputs (low bias current, no need for tight bias-Z matching at high source impedances), low noise, and bulletproof availability.
FAILURE MODES · 4
Saturation / clipping
If |Vin × 10| exceeds the supply rails minus a couple of volts of headroom, the output clips. On ±15 V supplies the TL072 will clip at about ±13 V, so input must stay below ±1.3 V to keep the output linear.
Capacitive load instability
Driving more than ~100 pF on the output (e.g. a long cable, a piezo, or a CMOS input through a long trace) introduces phase shift in the feedback loop and can cause oscillation or ringing on transients. Adds a small series resistor (50–100 Ω) at the output to isolate the load.
Missing supply bypass
Without C1/C2 on the supply pins, the rails sag during fast slewing and the output develops a peak-detected ripple. Worst at high signal frequencies and loud signal levels.
DC offset accumulation in cascades
The ~3 mV input offset is multiplied by 10 — at the output, that's 30 mV of DC. In a multi-stage chain this can stack up and eat headroom. Use AC coupling between stages where DC is irrelevant.
IMPROVEMENT SUGGESTIONS
◇ Lower-noise op-amp
Swap TL072 for OPA1612 or NE5532 if noise matters.
TL072's 18 nV/√Hz is fine for line-level signals but limits sensitivity in microphone or low-impedance sensor work. OPA1612 brings it down to 1.1 nV/√Hz at the cost of higher bias current and price.
◇ Compensation cap across Rf
Add a small (3–10 pF) capacitor in parallel with Rf.
Forms a low-pass at the desired bandwidth limit and rolls off out-of-band noise and any tendency toward HF oscillation. Trade-off: extra capacitance reduces large-signal slew capability.
◇ Rail-to-rail variant
For single-supply operation (e.g. 5 V or 3.3 V), use a rail-to-rail op-amp like the MCP6022 or OPA340 and bias V+ to mid-supply.
TL072 needs at least 1.5 V of headroom from each rail; on 5 V single-supply that leaves only ~2 V of swing. RRIO parts recover nearly the full rail.
[ END OF ANALYSIS ]
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