Design Ideas

Two-quadrant analog multiplier uses no log amps

Giovanni Romeo, Istituto Nazionale di Geofisica, Rome, Italy

You usually obtain the product of two values by summing the outputs of logarithmic amplifiers. The block diagram in Figure 1 shows a configuration that produces a two-quadrant product of analog signals but uses no log amplifiers. V_A and V_B are the multiplier inputs. The triangle generator produces a linear triangle whose voltage range is between 0V and the maximum value that V_B can attain (V_B>=0V). V_A may assume both positive and negative values. Figure 2 shows the circuit waveforms.

The comparator allows the V_A signal to reach the RC lowpass filter only when V_B exceeds the amplitude of the triangle wave supplied by the generator. Neglecting the V_A and V_B variations during the interval T, the lowpass-filtered signal at the output is:

However, from Figure 2, the value of T_ON is

So, combining Equations 1 and 2, we obtain the output value

The circuit produces the product of the two input signals. It performs well, using ordinary, low-cost components. Its limitations stem from the RC lowpass filter, which must suppress the triangle-wave frequency without affecting the signal (V_A and V_B) frequencies. Consequently, you should select a triangle-wave frequency that is much higher than the signal frequencies. (DI #1928)