Design Ideas: September 12, 1996

Standard linear output current for high-speed op amps is approximately 30 to 40 mA. When more current is necessary, the next available op amps provide approximately 200 mA but with a significant increase in cost and quiescent current. Depending on the amount of current you need, you may find it more cost-efficient to configure a dual op amp to provide adequate output current.

InFigure 1a , overall gain is 1+(R₂/R₁), and the output current is I₁+I₂. This configuration is acceptable only at low frequencies, such as audio, in which the intrinsic delay of the op amp is negligible with respect to the wavelength of the input signal. At higher speeds, the output of IC_1A significantly drives the load before IC_1B, and the output drive never reaches twice the current.

In high-frequency applications, the circuit in Figure 1b is preferable. In this case, the two op amps work in parallel. Because the dual package ensures a similar delay through each op amp, there is no fighting at the output, and the circuit delivers twice the current to the load. The overall gain is still 1+(R₂/R₁).

R_BM1 and R_BM2 are back-match resistors. When driving a transmission line in parallel, R_BM1 and R_BM2 should equal twice the characteristic impedance of the line to terminate the line properly. If proper matching to a load is unnecessary, then R_BM1 and R_BM2 simply isolate the outputs of IC_1A and IC_1B, limiting the current from any output offsets. For example, if the offset voltage is 10 mV between the output of the two op amps, then R_BM1=R_BM2=5[ohm] would limit the offset current to 1 mA. (DI #1924)