WIDEBAND_CURRENT_FEEDBACK_AMPLIFIER

Published:2009/7/13 1:45:00 Author:May | From:SeekIC

When using multipliers such as the LM1496 or MC1496, low-value output resistors are necessary to obtain maximum bandwidth. This reduces the output swing available, which necessitates a differ-ential high-gain, wideband amplifier. The amplifier has a differential gain (AV,diff) of 50 and a band-width of about 50 MHz, giving it a total gain-bandwidth product of 2.5 GHz. It also provides an output swing of 18 V p-p from a 112-V supply. Transistors Q1 and Q2 form a differential pair that drives Q3 and Q4. Feedback is provided to the emitters of Q1 and Q2 by R6, C1 and R7, C2, which bootstraps the input impedance and sets the overall gain. Q5 and Q6 provide about 15 mA of current each, pro-viding most of the sourcing current for the load. The basic design equations are provided to modify component values to suit different applications.Simply envision the circuit is as a differential current-feedback amplifier, with the low-imped-ance port being the emitters of Q1 and Q2, and the current output as the collectors of Q3 and Q4.Because the output is a bridge circuit and the maximum positive current is set by R6, R7 and Q5, Q6, the output is short-circuit-protected between resistors R8 and R9. RI can be replaced by a current source to reduce the common-mode gain. The main criterion is to balance the currents at the emit-ters of Q1 and Q2 to give a common-mode output voltage of VCC/2. Some care should be taken when driving capacitive load because the circuit can oscillate under such conditions.



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