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I haven't heard the term "digital transistor" vs. "analog transistor".
Anyway, a good transistor used in digital circuits (inside IC) should have as fast switching time as possible. A to state any good features of a transistor used in analog circuits depend heavily of the use of the transistor.
In a digital application, a transistor is operated in only 1 of 2 possible states...fully on (saturation) or fully off (cutoff).
When used in an analog application, the base-emitter junction is biased so that the transistor is in its linear region (you could say "partially on"). Typically you would bias it so that there's only enough base current flowing for the collector voltage to be pulled down to only 1/2 the supply voltage.
When you apply an alternating signal to the base, the positive going swing of the input signal causes more base-emitter current to flow, which allows the transistor to pull the collector end of the load resistor down towards the reference, which causes the load resistor to drop more of the total voltage while the transistor drops less of the total voltage.
The negative going swing of the input signal causes less base-emitter current to flow, which reduces collector-emitter current and allows the collector load resistor to pull the collector voltage up toward the supply rail. On this swing, the transistor would be dropping more than 1/2 the supply voltage while the load resistor drops less than 1/2 the supply voltage.
This causes the collector voltage to fall and rise as the input signal rises and falls. Adding a coupling capacitor to the collector with a pull down resistor on the opposite end of the capacitor blocks the static DC voltage that is present at the collector while the collector voltage alternations pass right through it and become your amplified output signal in the form of AC.
As far as an "analogue transistor" and a "digital transistor" I don't think there is such a thing. Any transistor AFAIK can be biased into its linear region and operated in an analog fashion. It's all in how it's biased.
At one time some bipolar transistors (and perhaps they are still made) were specifically designed for fast switching. They used gold doping to reduce the minority carrier lifetime and thus the turn-off saturation delay (storage) time. General purpose bipolar transistors without this doping have a much longer delay in coming out of saturation. The turn-on time is about the same with or without the doping.
So you have "digital" transistors with doping to kill the minority carrier lifetime, and then all other transistors without such doping, which could be considered "analog" types. But either type of transistor will work in either an analog or digital (switching) application, within their limitations.
FET transistors, being majority carrier devices, do not suffer from this saturation delay.
There does exist a component called a "digital transistor" Rohm is a major supplier. They are a transistor (SMD package) with base resistors integrated into the package. The resistor values are set to provide saturation of the transistor with a 5 Volt input. They are available with different configurations of the resistors. The idea is to further reduce the number of components needed in a product that needs to be small. We used these at a company where I worked to get more circuitry into a smaller PCB. They work OK. Google "digital transistor"
Basically it saves 2 resistors on the PC board. A logic level drives the transistor into saturation (hopefully) without the external need for external biasing resistors. This saves the manufacturer lots of PC board "real estate" and as we all know "real estate" is expensive.
Basically it saves 2 resistors on the PC board. A logic level drives the transistor into saturation (hopefully) without the external need for external biasing resistors. This saves the manufacturer lots of PC board "real estate" and as we all know "real estate" is expensive.
They're just optimized and characterized differently.
Transistors intended for digital use focus on optimizing and characterizing switching speed, how well current is blocked when off, and how well current is conducted when on, and other relevant parameters relevant for full on/off operation. They also don't care so much about noise or stability and linearity in the ohmic region.
Analog transistors are optimized and characterize for operation the ohmic region. They spend all their time operating halfway between on and off so they don't care about switching times or how well the transistors blocks or conducts when fully off or on. Instead, they care more about bandwidth, low noise and stable, linear, predictable, consistent behaviour in the ohmic region.
For example, since digital operation is full on or off, it means that if you make it very very efficient when fully on you can use less silicon. But this doesn't work for analog operation since operation in the ohmic region means you have to dissipate the same amount of power in the transistor no matter what for the circuit you use it in. It needs to dissipate that power to do its job, so you can't cheat and use less silicon by making it conduct better.
You can press one into service as the other, but since it may not work as well since it's not optimized for it and you may not have sufficient data to determine if it's appropriate since the relevant parameters where not characterized.
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