# Correct resistance for a transistor?

How do you know what R1 should be to completely turn on the transistor?

simulate this circuit – Schematic created using CircuitLab

• What do you mean by "turn on completely"? Do you mean you want the transistor in forward active or saturated mode? Jun 23, 2016 at 1:57
• I want saturated Jun 23, 2016 at 3:29

If by "turn on completely" you mean "produce 1 volt across R3", the answer is "none". As in "not going to happen." The best you'll get is about 0.3 volts, which corresponds to a 0.7 volt drop across the base-emitter junction. If you want 0.9 volts or better across R3, you'll need to change your circuit to

simulate this circuit – Schematic created using CircuitLab

then $$R1 = \frac{1 - 0.7}{10 R3}$$ as a first approximation. This works from the assumptions that a) the transistor can be driven into saturation with a Vce of less than 0.1 volts, and b) providing a gate drive of 1/10 the collector current will produce saturation, which is equivalent to saying that the gain of the transistor in saturation is 10, which is a standard rule of thumb.

• I think typically its Vce<0.2 for saturation? Jun 23, 2016 at 10:13
• @MaMba - For a 2N3904, 0.2 volts is a max specification for Ic of 10 mA or less. Typical numbers are more in the vicinity of 0.1. Jun 23, 2016 at 11:27

For the common collector configuration shown in your figure, transistor Q1 cannot saturate due to (a) the voltage developed across resistor R3 which causes negative feedback which limits $V_{BE}$, and (b) the "hallmark" of BJT saturation is $V_{E} < V_{B} > V_{C}$, and by inspection the condition $V_{B} > V_{C}$ cannot happen.

• If I use a transistor with leds, should I use a different arrangement so it will be saturated or should I use my circuit? Jun 23, 2016 at 15:48
• Use the common-emitter topology shown in @WhatRoughBeast's answer to your question. Connect the transistor's emitter directly to ground, and replace resistor R3 (in WhatRoughBeast's figure) with a series combination of your LED and a so-called current limiting resistor--i.e., a resistor that limits the amount of current that flows through the LED. Jun 23, 2016 at 20:09