Consider the Wilson current-mirror circuit of Fig. 7.34 when...

Consider the Wilson current-mirror circuit of Fig. 7.34 when supplied with a reference current $I_{\mathrm{RFF}}$ of $1 \mathrm{mA}$ What is the change in $I_{0}$ corresponding to a change of $\mathrm{tr}$ $+10 \mathrm{V}$ in the voltage at the collector of $Q_{3} ?$ Give both the absolute value and the percentage change. Let $\beta=100$ and $V_{A}=100 \mathrm{V}$

Key Concepts

Output Impedance

Output impedance in current mirror circuits determines how much the output current changes in response to changes in the output voltage. A high output impedance means that the mirror is less affected by variations in the collector voltage, which is a desirable feature in precision current mirrors like the Wilson configuration. The finite output impedance is a direct consequence of the Early effect and the non-ideal characteristics of the transistors used.

Transistor Beta (Current Gain)

Transistor beta, or current gain, is a measure of the ratio of collector current to base current in a bipolar junction transistor. In practical circuits, a finite beta means that some error is introduced by base currents, influencing the accuracy of current mirrors. In the Wilson current mirror, this concern is mitigated by circuit design that reduces the impact of base currents on the overall current replication.

Wilson Current Mirror

The Wilson current mirror is a variant of the basic current mirror that sends a reference current to a load with greatly improved current matching and increased output resistance. By using an extra transistor, it compensates for the base current losses that typically lead to errors in conventional current mirrors, resulting in more accurate current replication and reduced sensitivity to output voltage variations.

Early Effect

The Early effect is the phenomenon in bipolar junction transistors where a change in the collector-to-base voltage alters the effective base width, in turn causing the collector current to vary. This effect is characterized by the Early voltage (V_A) and leads to a finite output impedance; the larger the Early voltage, the less sensitive the current is to collector voltage changes. This is a key factor when analyzing changes in the output current due to voltage variations.

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