Differential Amplifiers Questions and Answers

Answer: B

Resistors in the two emitters apply local feedback exactly as they do in a single-ended stage, reducing the gain and extending the range over which the transfer is straight. A larger tail current raises the transconductance and narrows the range in millivolt terms, larger loads raise the gain, and common-mode range is a separate specification. Trading gain for linearity through series feedback is the same move in every topology. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: C

The two exponential characteristics combine into a hyperbolic tangent, which is linear near the origin and saturates at full steering in both directions. A field effect pair follows a square-law-derived shape instead, and neither a straight line nor a bare exponential describes it. That the saturation is smooth rather than abrupt is what lets the same circuit serve as a linear amplifier near the origin and as a limiter further out. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: D

The transconductance is proportional to the tail current, so a signal applied differentially is multiplied by whatever the tail current is made to represent - which is the basis of the Gilbert cell and of every analogue multiplier and balanced mixer. Offset, input impedance and output arrangement are not what makes it work. A stage whose gain is set by a current is a multiplier waiting to be used, and recognising that turns one circuit into a whole family. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: A

Driving the tail with one signal and the bases with another multiplies them, producing sum and difference frequencies while suppressing both inputs at the output - the frequency conversion every superheterodyne receiver needs. Amplification, comparison and regulation are other uses of related circuits. That suppressing the inputs at the output falls out of the balance, rather than needing a filter, is why the arrangement is called balanced and why it displaced the simple mixer.

Answer: B

Each device now carries a milliamp rather than half of one:

\[ g_{m} = \frac{I_{C}}{V_{T}} \propto I_{C} \]

so the transconductance and hence the gain double, for the same collector loads. The linear input range narrows in proportion, and the dissipation doubles. One parameter change moving gain, linearity and power in predictable directions is what makes the tail current the principal design lever in this stage.

Answer: C

Input-referred offset is multiplied by the gain on its way to the output:

\[ 100 \times 10^{-6} \times 1000 = 100 \ \mathrm{mV} \]

giving 100 millivolts, which in a high-gain stage can be a large fraction of the available swing. That a microvolt-level imperfection becomes a substantial output error is exactly why offset is referred to the input, and why it is the specification that limits precision direct-coupled measurement.

Answer: D

Referring an imperfection to the input divides out the gain, so two amplifiers of different gain can be compared on the same footing. It is actually harder to measure there, which is why it is computed from an output measurement. The mismatch does occur at the input but that is not why the convention exists. Referring an error to the input is a general technique that works because everything at the input is multiplied by the same gain as the signal. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: A

Pinch-off voltage describes a junction field effect transistor's channel closing and is a device parameter rather than a property of the pair. The other three are standard specifications for any differential input stage, bipolar or field effect. Spotting a parameter that belongs to one device family, appearing in a list of circuit-level specifications, is a quick way to identify the odd one out.

Answer: B

This is the common-mode rejection ratio, and it should be large - a hundred decibels is good. Conversion describes what a mismatch does when it turns common-mode into differential, transfer would suggest the opposite of rejection, and feedback is a different concept entirely. A figure of merit whose name says what the circuit does well, and which grows as the circuit improves, is one worth reading literally.

Answer: C

Equal inputs should give equal collector currents, so an output offset means the two halves are not matched - different devices, different loads, or a fault in one side. A failed tail source would leave both collectors at the supply, an exceeded common-mode range needs the inputs moved away from earth, and oscillation would show as an alternating output. Reading the resting condition before applying any signal is the first step, and it separates matching faults from everything else.