PROBLEM 063 · Beginner · Negative feedback sets the gain
Times Ten
Non-inverting amplifier: gain of 10
The problem
Build a non-inverting amplifier: the input source named “inp” drives the op amp’s non-inverting input, a resistor from “out” to “inn” (R1) feeds the output back, and a resistor from “inn” to ground (R2) completes the divider. Choose R1 and R2 for a closed-loop gain of 10 (20 dB).
With a large enough loop gain, the inverting input follows the non-inverting one, so the output is (1 + R1/R2) times the input. The op amp’s own gain only has to be large; its exact value no longer matters. The same feedback also reduces the bandwidth: GBW / closed-loop gain.
The test bench
- The test bench supplies the op amp as a ready-made block with terminals on the nets named “inp”, “inn” and “out”: DC gain 80 dB, one pole at 1 kHz (unity-gain frequency 10 MHz), output resistance 100 Ω.
- The test bench applies the AC stimulus to “inp”.
Numbers to hit
- Closed-loop gain between 19.7 dB and 20.2 dB
- Bandwidth (−3 dB) at least 0.8 MHz
- Peaking at most 0.5 dB
Design it yourself
Draw the circuit on a schematic canvas, size every transistor, and see it simulated in your browser with ngspice and the SkyWater SKY130 models. Every transistor lights up green, gold or red by operating region as you move the sliders.
Sign in or create a free account →More on feedback and stability
- Short on GainWhen the op amp’s gain is not huge, the textbook formula is off by several percent. Correct for it.
- Minus TenFeed the signal through a resistor into the inverting input: a virtual ground and a gain set by two resistors.
- Ramp MakerReplace the feedback resistor with a capacitor: the output is the integral of the input, with a gain that falls at 20 dB per decade.
- Light CatcherTurn a current into a voltage with one resistor, then stop the input capacitance from making it ring.
- No Ringing ZoneA capacitive load and a weak output stage make a second pole. Cancel its effect with a lead capacitor across the feedback resistor.