Bifet Variable Gain Amplifier With Maximized Control Voltage Range
Original Publication Date: 1989-Aug-01
Included in the Prior Art Database: 2005-Jan-28
This circuit is used in the read data path of a direct access disk drive. The analog disk signal from the head pre-amp output must be normalized to a constant amplitude, independent of disk signal amplitude variations. A Variable Gain Amplifier (VGA) in a feedback loop is used to perform the amplitude normalization. The variable gain is obtained by modulating the gate to source voltage of an NFET and, therefore, its channel resistance. The NFET device is connected between the emitters of two NPN transistors. Because of the other circuit design tradeoffs, the source and drain voltages of the NFET, at normal gains, vary statistically and with temperature. An external feedback loop around the VGA compares the output AC level against the desired level and generates an error voltage.