Can someone please explain to me what the purpose of this resistor is?
Electronic – the purpose of this resistor between BJT Base and Emitter
bjtcircuit-designtransistors
Related Solutions
One major parameter which decide biasing in BJT transistors is Bias Stability. As β (hFE) widely varies from transistor to transistor. An stable biasing will provide minimum alteration in the Q-point on wide changes in β.
Mathematically stability factor is denoted by,
S= delta Ic / delta Icb.
S depends on the circuit configuration and the bias resistors. S should be as small as possible.
Consider the thress biasing,
Now you can choose a biasing with low value of 'S' for better stability.
We never desire high value of 'S'. If one of the transistor stop working in your design then it's replacemnt may not have the same Beta. You want least effect on the Q-point due to the error in beta value. So need a low value of 'S'.
Consider these calculations:
VCEQ changes by 41% when β changes by 50%.
VCEQ changes by 25% when β changes by 50%.
VCEQ changes by 6% when β changes by 50%.
R2 is used to prevent a floating base. It gives it a defined state, in case the node labeled 2.8V isn't connected. It's a weak pull-down resistor. A floating pin, not pulled to a known state, will act like a mini-antenna, and can float high or low many times and turn the transistor on and off at random.
If that node is driven all the time, either high or low, then R2 is superfluous and can be removed. If the node is connected to for example a microcontroller gpio, that can go High-Impedence/Input (likely at start-up), then R2 keeps the transistor off until the microcontroller goes into output mode.
If the transistor is actually a Mosfet, then R2 is a small drain resistor. Mosfets have a capacitance that may keep it on, if not drained.
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Best Answer
That resistor speeds up turnoff and increases maximum allowed collector circuit voltage.
The turnoff speeding is based on the fact that the charge which is stored to the BE junction is dissipated in resistor R2. The effect is substantial in pulse circuits.
There's some leakage from C to B and it can cause substantial unwanted base current at high operating voltages. R2 sinks that leakage. The allowed max voltages in datasheets are often specified with R2 inserted.
I guess your R2=100kOhm is randomly selected. Practical R2 is often 100...1000x smaller.
ADD a simulation to demonstrate the effect to turn-off:
Here 5V 500kHz square wave (V1, red) drives a switching transistor 2N3904. The design is quite bad, the turn-on at the rising edge of V1 is slow and the turn-off at the falling edge of V1 is even slower due the stored charge in the BE junction:
Turn-on becomes faster when R1 is smaller, but the turn-off gets huge 400ns extra delay. That delay is called "storage time" and a small one can be seen already in the previous image:
Inserting R2 reduces the delay, but also degrades the turn-on speed:
R2 isn't perfect fix, but its better than nothing. The next is not asked, but it can be interesting:
Good performance is achieved with clamping diode and speeding capacitor:
The transistor sinks its excessive base current via D1 and the capacitor C1 gives a boost during both slopes of V1. D1 must be a low voltage drop type such as Germanium or Schottky diode, 1N4148 is useless. Here the used type is Schottky diode BAS81.