PIN-diode driver provides fast edges, bipolar swing

PIN (P-type, Intrinsic, N-type) diodes are used extensively as switches, attenuators, and phase shifters in RF and microwave applications. (If you are not familiar with PIN diodes and their applications, click here and here for more information.) They provide low series loss and low distortion, and can handle high amounts of RF power with relatively low control voltages. Their drivers typically need the following properties: TTL, CMOS, or other logic interface; bipolar output swing (±3V, for this design); the ability to sink and source 10 mA to 25 mA of steady state current; provide spiking current with fast rising and falling edges; and have minimal propagation delay.

The circuit shown in Figure 1 satisfies all of these requirements.

(Click to enlarge image)
Figure 1: This circuit meets the multiple requirements for driving PIN diodes.

The heart of the circuit is a pair of AD8037 clamp amplifiers (U2 and U3 ), which are set to a noninverting gain of +4. The AD8037's unique input clamp feature allows extremely clean and accurate clamping. They linearly amplify the input signal, up to the point where the gain times the clamp voltages (Vch and Vcl ) is exceeded. With a gain of 4, and clamp voltages of ±0.75 V, the maximum output voltage will be ±3 V.

In this case, the output voltage will be four times the input voltage for inputs less than ±0.75 V, but will be clamped to ±3 V when the input signal is greater than ±0.75 V. This clamping feature allows for very fast recovery (typically less than 2 ns) in over-driven conditions. The clamp voltages (Vch and Vcl ) are derived by voltage dividers R2 , R3 , R7 and R8 .

The 74F36 XOR logic gate (U1 ) provides a complementary drive signal for the AD8037s (U2 and U3 ) with minimal propagation-delay skew between the two complementary outputs. Resistors R4 , R5 , R9 , and R10 provide level shifting of the TTL output to approximately ±1.2 V. This voltage is then fed to U2 and U3 via R10 and R12 .

By design, the inputs to U2 and U3 are intentionally overdriven, ensuring that the outputs will go into the clamped state. Thus, the output levels for the PIN diode driver are set to ±3 V. Resistors R16 and R17 limit the steady state current. Capacitors C12 and C13 set the spiking current for the PIN diodes.

This circuit can be operated up to 10 MHz, with excellent switching performance and a total propagation delay of 15 ns. The output voltage and current can be “tweaked” to fit different applications, by varying either the gain or the clamp voltages.

About the author
John Ardizzoni is an applications engineer for high-speed amplifiers at Analog Devices, Inc. (Wilmington, MA). He can be reached at .

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