Transistors vs Op-Amps

[terrax]

Hello. I discovered this forum while performing a search on Google. I was trying to figure out what the minimum input voltage of an op-amp is.

Such led me to a six-year-old thread on here. But the answer got me wondering... Why can an op-amp amplify any voltage, but yet a silicon transistor requires ~0.6V before it can begin to function?

What gives? What is the difference? I mean, is not an op-amp just a number of transistors and other components, all in one convenient package?

 

[btbass]

An op amp has a differential input, it amplifies the difference between the two inputs. The internal transistors still need biasing to turn them on.

 

[enjunear]

An op-amp is a collection of transistors set up as a differential amplifier. As btbass stated, a differential amplifier amplifies the difference between the input pins. The transistors inside the op-amp are partially turned on, so small variations between the input signals are immediately amplified. When there is no activity in the circuit, the internal transistors and circuits inside the op-amp still consume power, because they are biased on.

A single transistor is, well, a single transistor. It operates in three regions of operation: cut-off, linear, and saturated. In cut-off, the transistor dissipates no power, because the collector current through the device is "cut off" (zero). The reason for the 0.6-0.7 V minimum base-emitter junction voltage is explained by the difference in the conduction band energy levels between the two semiconductor materials that make up the base-emitter junction (silicon doped as N-type, and silicon doped as P-type). When the energy levels are made equal potential, by applying a positive offest voltage to the base, current begins to flow inside the device. The P-N naming convention also shows up in why they are called NPN and PNP transistors, it describes the semiconductor material "sandwich" that makes up that particular device.

To learn more about semiconductor physics, check out The Britney Spears Guide to Semiconductor Physics. :wink: