Design Ideas July 20, 1995

Full-wave rectifier operates to 200 MHz

Rea Schmid,
Comlinear Corp, Fort Collins, CO

Doubling or rectifying a continuous-wave signal by using diode bridges usually causes problems with low inputs or at high frequencies. However, with the use of a 1.2-GHz current-feedback op amp and low-capacitance Schottky diodes in the feedback path, the circuit in Fig 1 can double the input frequency from 100 to 200 MHz. Designing active rectifier circuits with op amps is a well-known technique. Applying this technique to high-speed, current-feedback amplifiers, however, requires slightly different design considerations.

A current-feedback amplifier's bandwidth and stability are dependent on the capacitance of the internal high-impedance node and the external feedback resistor, R_F. For a simple gain stage, you select the value of the feedback resistor by plotting open-loop gain vs closed-loop gain for the loop gain of the denominator in the following expression:

V_OUT/V_IN = [lalph](1 + R_F/R_G) / 1 + (R_F + R₁(1 + R_F/R_G)) / Z($)

where à is the buffer gain (about 0.999) and R_I is the buffer's input impedance at the negative node (about 25 Ohm).

In this plotting exercise, you need to select the feedback resistor's value to provide a closed-loop phase margin of 60ø at the unity-gain frequency. The foregoing expression shows that, for a low gain setting and low values of R₁, the amplifier's gain is independent of the gain-bandwidth product.

Placing Schottky diodes alone in the feedback loop is not acceptable for current-feedback amplifiers because doing so compromises unity-gain stability. You need to place resistors in series with the diodes (Fig 1). As the diodes conduct in each polarity, the diodes' resistance decreases to a low value while the series resistances, R_F1 and R_F2, maintain the required unity-gain stability.

Once you determine the diodes' on-resistance, r_D, you can select the dc gain for amplifier IC₁ by choosing a value for R_G from the following:

GAIN = 1 + R_F + r_D / R_G

To maintain a balanced ac load at the connection of the R_F1 and R_F2 resistors and the D₁ and D₂ diodes, a high-impedance differential amplifier (IC₂) To set the gain of the variable-gain differential amplifier, you select first R_G, then R_F from the following expressions:

R_G = V_{PEAK} / 1.8 mA

A_v = 1.85 x R_F/R_G [V_G + 1 / 2]

R_G sets the maximum common-mode input range for IC₂, and R_F sets the output amplifier's bandwidth. A voltage applied to the V_G input at pin 2 provides gain control, as the above expression shows. (DI #1735)

PICTURE 1

This circuit uses an old design technique with fast current-feedback amplifiers to configure a 100- to 200-MHz frequency doubler.