Electronic Design

The "Starved-Circuit" Amplifier Is Revived In A Transistor Version

Back in the '60s, there was an intriguing circuit article titled "Starved-Circuit Amplifier" published in Popular Electronics magazine. It used a sharp cut-off pentode with a 3-MΩ load resistor and had a voltage gain of over 1000. Of course, it had little application since any load added to the output tended to reduce the load resistance and the gain.

Like tube-type amplifiers, transistor amplifiers have a maximum voltage gain of about 100 (40 dB). Additional stages are required to increase the voltage gain. A cursory examination of this circuit might indicate that this amplifier uses a conventional composite npn-pnp transistor arrangement, which is nothing new (Fig. 1). What is new is the value of R4. In the typical composite connection, R4 is 50k, which is sufficient to drain off leakage to increase the dc stability. In this case, however, R4 = 270 (more than two orders of magnitude lower).

Q2's VBE regulates the current through R4. This current source is Q1's collector current. A good current source has the property of an infinite ac resistance, which increases the voltage gain of Q1 by a factor of 10. The typical voltage gain is about 1500 (63 dB).

Q2, operating as an emitter-follower, has a unity voltage gain. To prevent the dc bias circuit from providing ac feedback, C2 bypasses the ac to common. For best results, Q1 should have a beta that exceeds 250. Q2 should have a relatively high beta as well.

High voltage gain is just one of the beneficial properties. Low distortion is another. Since the ac load line is horizontal, IC doesn't vary over the large-signal voltage range. As a result, large-signal distortion is low even without feedback. Noise performance is good because Q1 runs at about 2.5 mA, which is close to the minima of the noise curve. The equivalent noise input voltage runs below 1 µV. Another feature of this circuit is that the frequency response of its amplifier is suitable for audio applications. The input resistance runs at about 5k, while the output resistance, driven by an emitter-follower, is low.

This circuit may be redrawn as an operational-amplifier gain block with external components controlling the gain. An alternative hookup uses a resistor in the ground-return circuit. In this "current feedback" version, the voltage developed across R8 is a voltage feedback signal (Fig. 2). It has the advantage of reducing gain, noise, and distortion while increasing input resistance (Fig. 3).

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