Electronics > Beginners
NPN Transistor as a switch: what's the use of the emitter resistor?
KE5FX:
--- Quote from: bson on March 21, 2019, 12:08:42 am ---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.
--- End quote ---
Win Hill actually just posted a nifty example of this practice as an appetizer for the 'X-chapters' volume for AoE 3. Some grad students needed to dump 200 amps into an LED in a hurry, and they came to him for advice when their first attempt failed to achieve the leading-edge speed they needed. It made for an interesting case study.
Arguably, he's not trying to avoid saturation on the transistor's part, so much as he's trying to ensure that the LED sees the same current across the entire pulse as it heats up over time. Similar basic motivation, though.
Zero999:
As far as using this circuit to switch a load, rather than as a linear amplifier, the only thing I can think of is an NPN transistor controlling the current to a PNP transistor's base, or the voltage across a MOSFET's gate, which in turn switches the load.
R1 holds Tr1 off, preventing any leakage through Tr2 from turning it on. It also helps to speed up the off time of TR1 by discharging its base, when Tr2 is turned off. It be a fairly low value, say 1k.
VR2 = VINPUT - VBE
IR2 = VR2/R2
Assuming Hfe for Tr2 is big and hardly any current flows through R1
Tr1_IB = IR2
Example:
Suppose we need to switch a 200mA load and Tr1 has a low saturation voltage when IB = 1/20 IC. The minimum input voltage is 4.5V
IC = 0.2A
IB = 0.2/20 = 0.01A
Calculate R2
IR2 = IB = 0.01A
VBE = 0.7V
VR2 = VINPUT - VBE = 4.5-0.7 = 3.8V
R2 = 3.8/0.01 = 380R. Use 330R.
Peabody:
Another application using the emitter resistor would be to have the base driven by the PWM ouput of an MCU GPIO port pin, with appropriate filtering. The duty cycle of the PWM (the average voltage applied to the base) would control the current through the load - i.e., the brightness of an LED.
bsfeechannel:
--- Quote from: Jwillis on March 20, 2019, 08:37:26 am ---Emitter resistor is a way of biasing the transistor to prevent thermal run away.
--- End quote ---
I'll bet on this horse.
jlmoon:
The original circuit looks to be a common emitter type amplifier minus the biasing scheme on the base connection. Not much of a switch unless you take out the emitter resistor, tying the emitter to ground and put the load in series (ie: small relay or such) with proper current limiting in the collector circuit.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version