differensialpar

Differensialpar, or differential pair, is a basic transistor circuit used to amplify the difference between two input signals. It typically consists of two transistors with their emitters (BJTs) or sources (MOSFETs) tied together to a tail current source, forming a long-tailed pair. The collectors or drains are connected to power through loads, which can be resistors or active devices, and the output can be taken differentially (from both collectors/drains) or single-ended (from one side).

Operation is governed by the differential input voltage. The tail current is constant, so as the input difference grows, more current shifts from one transistor to the other. For BJTs, the collector currents follow I_C1 = I_tail/2 [1 + tanh(v_diff/(2V_T))] and I_C2 = I_tail/2 [1 - tanh(v_diff/(2V_T))], where v_diff is the input voltage difference and V_T is the thermal voltage. Small-signal behavior yields a differential gain A_d ≈ g_m R_C, with g_m ≈ I_tail/(2V_T). In MOSFET implementations, g_m is set by bias currents and device overdrive.

Key characteristics include high input impedance at the bases or gates, good common-mode rejection when properly biased, and flexibility in using active or passive loads. Practical designs emphasize transistor matching, temperature stability, and linearity, sometimes improved by emitter degeneration or using current mirrors as active loads.

Applications span the input stage of operational amplifiers, instrumentation amplifiers, comparators, and analog-to-digital converters. Variants range from bipolar long-tailed pairs to purely MOS differential pairs, often integrated with additional circuitry to enhance gain, bandwidth, and noise performance.