Challenge your understanding of differential amplifiers and current mirrors, key concepts in analog circuit design. This quiz covers operational principles, biasing, applications, and typical configurations associated with these essential building blocks.
In a differential amplifier using an NPN transistor pair with a current mirror as the tail source, what mainly determines the common-mode rejection ratio (CMRR)?
Explanation: The CMRR in a differential amplifier depends heavily on how closely the two transistors are matched in terms of their electrical characteristics. Mismatched transistors can lead to higher common-mode gain and lower CMRR. While a precise current mirror helps with biasing, it is not the dominant factor. Emitter degeneration can increase CMRR, but only to a certain extent, and the collector load resistance primarily affects voltage gain, not CMRR directly.
What is the primary function of a current mirror circuit in analog design, for example, when used as a load for a differential amplifier?
Explanation: Current mirrors are used to generate a constant current regardless of voltage changes at their output, making them ideal biasing elements in analog circuits like differential amplifiers. They do not amplify voltage or invert signals. Increasing input impedance is sometimes a secondary effect, but the primary role is constant current sourcing or sinking.
Suppose the power supply voltage increases in a differential amplifier with a current mirror tail. What most likely happens to the tail current if the reference voltage of the current mirror remains constant?
Explanation: A properly designed current mirror sets the tail current based on its reference voltage and resistor, making it relatively insensitive to changes in the power supply. The tail current only varies slightly with supply voltage changes. If the current mirror is working correctly, options such as 'increases sharply' or 'decreases significantly' are incorrect.
If the inputs of an ideal differential amplifier receive two identical voltages of 1.5 V each, what would be the output signal?
Explanation: An ideal differential amplifier amplifies only the difference between its two inputs. If both inputs are the same, the difference is zero, resulting in zero output voltage. The other options incorrectly assume the amplifier reacts to common-mode signals or produces a static output.
Why might emitter degeneration resistors be added to the emitters of a differential amplifier’s transistors?
Explanation: Emitter degeneration resistors help linearize the input-output relationship and increase the common-mode rejection ratio, making the amplifier more stable and precise. They do not directly reduce collector power dissipation, affect the current mirror output, or significantly lower input bias voltage. Those options misunderstand the main advantage of emitter degeneration.