Let the NPN transistors have beta= 150 and Early voltage V A = 75V. Let the PNP

Question

Let the NPN transistors have beta= 150 and Early voltage VA = 75V.

Let the PNP transistors have beta = 120 and Early voltage VA = 50V.

Use the Miller effect to estimate the break frequency of the dominant pole of the open-loop gain.

Notes:

Assume that Ic6 and Ic7 and equal when computing the transconductance of Q6 and Q7. (In practice, Ic7 will be ~ 2.1mA greater than Ic6 so as to provide a base current to Q3, but the error introduced by the assumption that the currents are equal is not large).

Include the base-emitter small-signal resistance rpi3 in parallel with r02 and r07 when computing the voltage gain of the differential pair with current-mirror load. Remember that rpi= beta/gm.

The output stage consisting of Q4, Q5, Q11, Q12, R7, and R8 is a unity-gain buffer for driving low-resistance loads. You may ignore it in computing the differential-mode voltage gain.

Explanation / Answer

There are two amplifier circuits below. The first uses a traditional op amp (modeled as a voltage-controlled voltage source) which is assumed to be ideal except for limited bandwidth. The second uses a device known as a current-feedback op amp which is modeled as a transresistance amplifier, also assumed to be ideal except for limited bandwidth. <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Assume that the open-loop voltage gain of the traditional op amp is A(s) = Avo ?o / (s + ?o) where dc open-loop gain Avo = 40,000 V/V and open-loop break frequency ?o = 2?

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