CE / CS Amplifier Bench

Quick experiments

• Turn a gain into decibels. Set a voltage gain of 10 and read 20 dB. Gain 100 gives 40 dB, gain 1000 gives 60 dB — every ×10 in voltage adds 20 dB, because dB is a logarithmic scale.
• Drive the amplifier into clipping. With a 12 V supply and a gain of 20, raise the input past 300 mV peak. The required 6 V peak output no longer fits inside the rails and the waveform tops flatten.
• Trade gain for bandwidth. For a fixed gain-bandwidth product, halving the gain doubles the usable bandwidth. A 1 MHz GBW part gives 1 MHz at unity gain but only 10 kHz at a gain of 100.
• Stack two stages. Cascade gains of 10 and 20 for an overall 200. In decibels that is 20 dB + 26 dB = 46 dB — gains multiply, but decibels simply add.
• Check that a gain below 1 is attenuation. Set the gain to 0.5. The result is −6 dB. Negative decibels mean the signal shrank, which is exactly what a divider or a pad does.

Formula reference

Voltage gain

$$A_v = \frac{V_{out}}{V_{in}}$$

A dimensionless ratio, not yet in decibels.

Voltage gain in decibels

$$A_{dB} = 20 \log_{10} A_v$$

×10 → 20 dB, ×2 → about 6 dB, ×1 → 0 dB.

Power gain in decibels

$$G_{dB} = 10 \log_{10}\!\left(\frac{P_{out}}{P_{in}}\right)$$

Uses 10, not 20, because power already varies as voltage squared.

Gain-bandwidth product

$$GBW = A_v \times f_{-3dB}$$

Roughly constant for a given op-amp, so gain and bandwidth trade directly.

Common mistakes

• ✗Using 10·log for a voltage ratio.

  Voltage and current ratios use 20·log; power ratios use 10·log. Applying the power formula to a voltage gain of 10 gives 10 dB instead of the correct 20 dB.

• ✗Adding gains from cascaded stages.

  Plain gains multiply — stages of 10 and 20 give 200, not 30. Only the decibel values add, which is precisely why designers work in decibels.

• ✗Expecting full output swing right up to the rails.

  Most op-amps lose one to two volts at each rail. A 12 V supply may only swing 8–10 V peak-to-peak unless the part is specified rail-to-rail — and even then only lightly loaded.

• ✗Quoting bandwidth without stating the gain.

  Bandwidth is meaningless on its own. A 1 MHz GBW amplifier has 1 MHz of bandwidth at unity gain but only 10 kHz at a gain of 100.

• ✗Blaming distortion entirely on clipping.

  Slew-rate limiting distorts fast signals long before the peaks flatten. If a large-amplitude sine turns triangular as frequency rises, the slew rate is the limit, not the supply.

Frequently asked questions