PROBLEM 103 · Easy · A fault in: differential pair with resistor loads
Tilted
A pair that is not balanced
The problem
The circuit on the canvas was built for the problem “Differential pair with resistor loads”, but it does not meet the specification. With both inputs at the same voltage the two outputs should be equal, but they are far apart.
Find what is wrong and repair it: change a value, move a wire or add a part. Everything the original problem asked for still applies (the targets are listed below), and the live readings and the colours of the transistors will show you where to look. Check your circuit when you think it is fixed.
The test bench
- Supply: VDD = 1.8 V.
- The test bench hangs a 200 fF load capacitor from each of “outp” and “outn” to ground.
- It applies the AC stimulus to the source named “inp” only, and measures the differential output, the difference “outp” minus “outn”.
Numbers to hit
- Differential gain at least 12 dB
- Bandwidth (−3 dB) at least 40 MHz
- Output common-mode level between 0.8 V and 1.5 V
- Output DC imbalance at most 20 mV
- Power at most 250 µW
- Every transistor in saturation
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.
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