EDN Access -- 07.07.94 Single resistor betters inexpensive buffer
-- EDN, July 7, 1994
Design Ideas:July 7, 1994
Single resistor betters inexpensive buffer
Jim Riphahn,
Comlinear Corp, Fort Collins, CO
Differential gain and phase are important video characteristics, and video-equipment manufacturers welcome any low-cost method of improving these parameters. Fig 1 shows one simple method using a low-power, low-cost, closed-loop buffer with a total circuit cost of around $1.60, including resistors and bypass capacitors. Because output-stage drive capabilities affect differential gain and phase, low power often translates into degraded numbers. For example, the CLC109's typical differential gain and phase are 0.7% and 0.03 degrees, respectively, while driving a 150 Ohms load. However, by adding a 1-kOhms pulldown resistor on the output pin, the respective numbers improve to 0.05% and 0.01 degrees, respectively. Thus, the cost of a single resistor brings an order-of-magnitude improvement and makes the differential gain and phase of the buffer comparable to much more expensive devices. However, if low power is a sensitive issue, you should carefully consider this method. Adding the resistor increases Fig 1's typical supply current from 3.5 to 7.5 mA. This idea works equally well with video op amps. (DI #1553)
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