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Improved Amplifier Drive Scheme

IP.com Disclosure Number: IPCOM000060817D
Original Publication Date: 1986-May-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 2 page(s) / 46K

Publishing Venue

IBM

Related People

Bond, PW: AUTHOR

Abstract

The circuit provides a means to increase the gain and output drive of an operational amplifier pre-driver stage without creating frequency stability problems associated with additional stages. The circuit of Fig. 1 shows an example of the prior art. Q1 through Q6 form a typical front end of an operational amplifier. The object of the pre-driver is to supply a large amount of current, IO, to an external power stage. Q1 and Q2 form a differential input, while Q3, Q4 and Q5 form a current mirror. The output is taken from the emitter of Q6 .When IO becomes large, the base current of Q6 becomes large, which forces an offset in the input Vin . The input offset is unacceptable. Making Q6 a darlington would reduce the base current; however, the darlington introduces additional phase shift, causing frequency stability problems.

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Improved Amplifier Drive Scheme

The circuit provides a means to increase the gain and output drive of an operational amplifier pre-driver stage without creating frequency stability problems associated with additional stages. The circuit of Fig. 1 shows an example of the prior art. Q1 through Q6 form a typical front end of an operational amplifier. The object of the pre-driver is to supply a large amount of current, IO, to an external power stage. Q1 and Q2 form a differential input, while Q3, Q4 and Q5 form a current mirror. The output is taken from the emitter of Q6 .When IO becomes large, the base current of Q6 becomes large, which forces an offset in the input Vin . The input offset is unacceptable. Making Q6 a darlington would reduce the base current; however, the darlington introduces additional phase shift, causing frequency stability problems. Increasing the bias current I would improve the base drive situation but would also degrade the frequency response of Q1, Q2 . The circuit of Fig. 2 considerably reduces the loading problem with the addition of current mirror devices Q8 and Q9 . Essentially, the output current IO is split in half through the action of Q8, Q9 . IO = 2I2 = 2I1 . If the base current of the Q3, Q4 mirror is small compared to I1, then the emitter currents of Q5 and Q6 are separately equal as is their respective base currents. With equal base currents, the loading on Q1 and Q2 is equal (equal collector currents), and the offset voltage of...