Give the circuit for the p np version of the basic current...

Give the circuit for the $p$ np version of the basic current mirror of Fig. $7.28 .$ If $\beta$ of the pnp transistor is $20,$ what is the current gain (or transfer ratio) $I_{O} / I_{\mathrm{REF}}$ for the case of identical transistors, neglecting the Early effect?

Key Concepts

Neglecting the Early Effect

The Early effect describes the phenomenon where changes in the collector-base voltage alter the effective base width, impacting the output characteristics of the transistor, notably its current. By neglecting the Early effect, one assumes an infinite output resistance for the transistor, thereby simplifying the analysis by focusing solely on the current mirroring and beta-related error components.

Finite BJT Beta Effects

The beta (?) of a bipolar transistor, defined as the ratio of collector current to base current, plays a significant role in current mirror accuracy. In a basic current mirror, some of the reference current is diverted to provide the necessary base current for both transistors. With a finite beta, this base current draw leads to an error in the output current relative to the reference—a key consideration when analyzing the transfer ratio of the mirror.

Current Mirror

A current mirror is a circuit configuration that copies or “mirrors” a reference current from one active device into another branch of the circuit. It is widely used for biasing and active load applications, relying on device matching to ensure that the output branch current tracks the reference current as accurately as possible.

PNP Transistor Operation

PNP transistors are bipolar junction transistors where the current flows from the emitter to the collector when the base is maintained at a voltage lower than the emitter. In a PNP current mirror, the orientation of the devices is inverted compared to the more common npn version, but the underlying principle of mirroring a current using matched transistors remains the same.

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