Topic Name: Full-wave rectifier uses current-feedback amps
Research persons: 4engr team
Location: Albama, United States
The circuit in Figure 1 uses current-feedback amplifiers to implement a wideband full-wave rectifier for applications such as a control/AGC system reference or as an amplitude indicator. Putting the full-wave-rectifier diodes in the feedback loop of an op amp works better than does using simple diode-based rectifiers. The op amp's loop gain decreases the distortion and offset voltage that the diodes introduce. Simple diode rectifiers also require large signals and are temperature-sensitive. The op-amp technique works well for signal frequencies to about 500 kHz, but at higher frequencies the loop gain of voltage-feedback op amps dramatically decreases, which causes the error to rapidly increase. Also, the error becomes frequency- and temperature-dependent, which can introduce nonlinearities into the system.
Current-feedback amplifiers have adequate high-frequency loop gain for this application but are unusable in standard full-wave-rectifier configurations because the amps usually oscillate unless they have a resistive-feedback element. However, you can take advantage of the high-frequency performance of current-feedback amplifiers if you configure the full-wave rectifier as Figure 1 shows. IC1 and IC2 are high-frequency current-feedback op amps with no pulldown transistors in the output buffer. Without these transistors, the op amp works normally when the output swings positive, but the output disconnects when it swings negative. As configured in Figure 1, the op amp amplifies positive inputs, and because the output disconnects for negative inputs, the termination resistors set the output voltage to zero when the input is negative. Thus, this configuration makes a good half-wave rectifier.
In the circuit, the input signal drives IC1 and IC3A. IC1 passes the positive swing, and IC3A inverts the complete signal. IC2 passes the positive half of its input signal, which is the inverted negative half of the input signal. R2, R3, R4, and R5 act as bias circuits, keeping the output from swinging below ground during negative input-signal excursions. This bias is necessary because the op amp's input buffer causes the inverting input to follow the noninverting input. R1 and R3 act as an attenuator, and R2 slightly pulls up the output voltage. The two positive half-wave-rectified waveforms combine through R6, R7, R8, and R9. You can adjust R7 and R9 to obtain equal amplitudes for each half of the full-wave-rectified signal. IC3B, a high-bandwidth op amp, amplifies the composite signal.
Figure 2a shows the input signal and the resultant rectified output for an input frequency of 1 MHz. Note that the input and the output have equal amplitudes and that the output has the characteristic sharp points on the bottom. If IC1, IC2, and IC3 were not high-bandwidth circuits, the points would be rounded on the bottom of the output signal and the amplitude would be smaller. With an input signal of approximately 10 MHz (Figure 2b), these points become round and the rectified signal starts to look like a double-frequency sine wave. This distortion is not harmful, because the sharp points contain no information. Also, because the apparent sine-wave voltage in Figure 2b goes from zero to +VPEAK, you can easily integrate this signal as an error signal.
As you continue to increase the input frequency beyond 10 MHz, the rectified output signal decreases in amplitude because of the high-frequency roll-off. However, the circuit is still usable because the frequency roll-off is predictable. If you adjust the circuit for the proper output signal at a particular operating frequency, the circuit yields excellent results beyond 25 MHz. (DI #2005)
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