High speed, high current gain voltage buffer and method

Abstract

An amplifying circuit which may be useful in a diamond buffer amplifier or operational amplifier includes an input transistor including an emitter, a collector, and a base coupled to receive an input voltage. An adjustable current source circuit is coupled between a first reference voltage and the emitter of the input transistor. A current source is coupled between a second reference voltage and the collector of the input transistor. An isolation resistor has a first terminal coupled to an output terminal of the adjustable current source circuit and a second terminal coupled to the emitter of the input transistor. A current follower circuit is coupled between the collector of the input transistor and an input terminal of the adjustable current source circuit. A feed-forward capacitor is coupled between the collector of the input transistor and the first terminal of the isolation resistor.

Description

BACKGROUND OF THE INVENTION[0001]The present invention relates generally to so-called diamond buffer amplifiers, and more particularly to diamond buffer amplifiers having substantially improved slew rate performance and high current gain at high frequencies.[0002]FIG. 1 shows a simple prior art diamond buffer amplifier 1A, which is similar to well-known basic diamond follower circuits. In FIG. 1, an input signal Vin is coupled to the bases of a PNP input transistor Q4 and an NPN input transistor Q3. The emitter of input transistor Q3 is coupled by conductor 5 to a constant current source I1 and also to the base of a PNP output transistor Q2. Similarly, the emitter of input transistor Q4 is coupled by conductor 4 to a constant current source I2 and also to the base of an NPN output transistor Q1.[0003]The current gain of diamond buffer amplifier 1A of FIG. 1 is βnpn×βpnp, which can be considered to be simply β2. In some cases, the current gain is too low because β2 is too low. Also, ...

AI Technical Summary

Benefits of technology

[0011]It is an object of the invention to provide buffer amplifier circuitry having both high slew rate and high current gain at high frequencies.

[0012]It is another object of the invention to provide buffer amplifier circuitry, having both high-slew-rate and high current gain at high frequencies, which can be used as a building block in amplifier circuitry.

[0013]It is another object of the invention to provide a diamond buffer amplifier having both high slew rate and high current gain at high frequencies.

Problems solved by technology

In some cases, the current gain is too low because β2 is too low.

Low values of βnpn and βpnp also set a limit to the ratio of the quiescent current of diamond buffer amplifier 1A to the maximum value of output current lout and therefore can limit the effectiveness of diamond buffer amplifier 1A.

Unfortunately, the foregoing frequency compensation technique shown in FIG. 3 does not allow the currents in controlled current source transistors Q5 and Q6 to rise very fast to immediately deliver the maximum current and rapid recharging of the parasitic base capacitances of output transistors Q1 and Q2.

As a result, the buffer amplifier 1C of FIG. 3 is not much better than the simple one shown in “Prior Art”FIG. 1 with regard to slew rate and current gain at high frequency.

Thus, there is an unmet need for buffer amplifier circuitry that provides a high slew rate and high current gain at high frequencies.

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