Design Ideas: December 7, 1995

The widest use of limiting amplifiers is for clamping analog signals within a desired range. However, limiting amplifiers with fast overdrive-recovery times and wide bandwidth have an interesting additional application: You can use these amplifiers to form AM modulators.

Typically, you provide dc voltages for the high and low clamp levels, and the amplifier limits its output range to those levels, regardless of the input overdrive. Properly designed limiting amplifiers have a fast overdrive-recovery time, which allows them to quickly return to linear operation when the input falls within the clamp levels. This fast overdrive-recovery time and wide bandwidth of the clamp inputs allow you to drive these inputs with high-frequency ac signals as well as dc signals. When you drive these clamp inputs at the appropriate levels, you can use the inputs to form an AM modulator.

Figure 1 shows a complete AM-modulator circuit. A 4V_p-p carrier signal drives limiting amplifier IC₂. The gain of 2 through IC₂ ensures that the carrier amplitude is sufficient to drive the output over its ±3.3V range. IC₁ performs the necessary level shifting and inversion to convert the modulating-signal input into a pair of antiphase signals that control the high and low clamp inputs. IC_1A inverts the signal and level shifts it to -1.5V. IC_1B inverts that signal to form a complementary signal centered at 1.5V.

With an input signal of 0V, IC₂ produces a 3V_p-p output at the carrier frequency. As the signal input varies, IC₂ produces a symmetrically modulated carrier with a maximum amplitude of 6Vp-p. The 2300-V/µsec slew rate of the HFA1130 limits carrier signals with amplitudes of 6V_p-p to a frequency of 61 MHz. With lower output-signal levels, you can increase the carrier and modulating frequencies to several hundred megahertz.

The scope photos in Figure 2 show the circuit's performance with a 1-MHz carrier and a 31-kHz signal. The circuit doesn't produce the spectral purity of a two-quadrant multiplier or mixer. However, compared with these approaches, the circuit's real advantage is bandwidth; it can perform amplitude modulation into the megahertz range. The trade-off with this bandwidth is higher harmonics, and you may have to do some filtering. Soft clipping in the amplifier helps to minimize harmonics. Also, note that there are classes of amplitude modulation, such as pulse-amplitude modulation, for which amplifier rise time is more important than spectral purity. (DI #1791)