0
\$\begingroup\$

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

schematic

simulate this circuit – Schematic created using CircuitLab

\$\endgroup\$
2
  • \$\begingroup\$ What do you mean by "turn on completely"? Do you mean you want the transistor in forward active or saturated mode? \$\endgroup\$
    – The Photon
    Jun 23, 2016 at 1:57
  • \$\begingroup\$ I want saturated \$\endgroup\$ Jun 23, 2016 at 3:29

2 Answers 2

3
\$\begingroup\$

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

schematic

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.

\$\endgroup\$
2
  • \$\begingroup\$ I think typically its Vce<0.2 for saturation? \$\endgroup\$
    – MaMba
    Jun 23, 2016 at 10:13
  • \$\begingroup\$ @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. \$\endgroup\$ Jun 23, 2016 at 11:27
0
\$\begingroup\$

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.

\$\endgroup\$
2
  • \$\begingroup\$ If I use a transistor with leds, should I use a different arrangement so it will be saturated or should I use my circuit? \$\endgroup\$ Jun 23, 2016 at 15:48
  • \$\begingroup\$ 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. \$\endgroup\$ Jun 23, 2016 at 20:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.