Circuit forms high-frequency polarity clamp

You need a negative signal clamp to protect an ADC against negative-going signals while measuring the positive pulse shape of the signal by digitizing the waveform. In other words, you must clamp negative signals at 0V. The circuit in Figure 1 can clamp either positive or negative pulses to ground. The circuit uses a fast MAX477EPA amplifier to drive the Q₁ or Q₂ to clamp signals of either positive or negative polarity. Q₁ forces negative signals to a 0V level; Q₂ does the same for positive signals. You use an spdt switch to choose the transistor of interest. The circuit works over 1 kHz to 10 MHz. You can also make the clamp operate at frequencies as high as 100 MHz by replacing C₁ with a 1-kΩ, 0.5W resistor. However, the output would need another MAX477EPA buffer amplifier to drive the 50Ω signal cable, because the circuit's output impedance increases to 1 kΩ.

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