Electronics > Beginners
NPN Transistor as a switch: what's the use of the emitter resistor?
magic:
This isn't guaranteed to prevent saturation, Baker clamp should be used for that.
Benta:
--- Quote from: magic on March 20, 2019, 06:20:52 pm ---This isn't guaranteed to prevent saturation, Baker clamp should be used for that.
--- End quote ---
Correct. But it depends on the load, if it has too high resistance, saturation may indeed occur.
David Hess:
As shown in Moriambar's example, the emitter resistance limits the current to (Vin-Vbe)/R2 where Vbe is about 0.6 volts.
Base series resistor R1 has only a minor effect because the transistors current gain (hfe) should be much greater than the ratio between base series resistor R1 and emitter series resistor R2. It however should be included because if the load is missing, then the drive signal will see an effective short to ground through the emitter series resistor.
Nothing about emitter series resistor R2 prevents the transistor from going into saturation if the collector voltage falls and again, this is why R1 is included to protect the driver.
Moriambar:
--- Quote from: Audioguru on March 20, 2019, 04:28:03 pm ---The transistor circuit shown is called a "phase splitter". The phase at the emitter output is the same phase as the base input but the phase at the collector output is -180 degrees.
--- End quote ---
The output (shown as RL) is at the collector, not at the emitter
bson:
The emitter resistor makes it a current switch rather than a voltage switch. Some loads, particularly semiconductors but also solenoids, are current operated and need a particular current rather than voltage. With an emitter resistor you don't want a base resistor; base current is low as the resistance looking into the base is RE*hfe = high for high hfe, and a base resistor just makes life more complicated. You don't use a collector resistor with a current switch.
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