# Do I need base resistor when it's tied to a collector resistor from previous stage?

#### tpny

Joined May 6, 2012
216
do I need a resistor in ??? below?

Rich (BB code):
           vcc
|
resistor
|
|---------------???----------------transistor base
/
input----resistor----|  npn
\
|
gnd

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Joined Dec 26, 2010
2,148
I would say not necessarily, it all depends on the supply voltage, the values of resistance used, and the ratings of the transistors used.

For instance, the current flowing into the second transistor base with the first transistor turned off must nt be too great. Also, the first transistor must be capable of turning on to a sufficiently low VCE voltageto turn the second transistor bias current low enough. That said, simple arrangements like this are seen in practical circuits.

A low - value base resistor at the second transistor base is also sometimes needed as a means of improving high frequency stability, sometimes called a base "stopper".

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#### Ron H

Joined Apr 14, 2005
7,014
A low - value base resistor at the second transistor base is also sometimes needed as a means of improving high frequency stability, sometimes called a base "stopper".
Base stopper resistors (I had never heard the term, but I have used them many times) are not required for stability in saturating stages, such as that posted by tpny. They are typically needed in emitter followers which have a small amount of cap loading on the emitter, and some possible parasitic inductance in the base circuit. The base of an emitter follower sometimes exhibits negative resistance at high frequencies. If the resonant frequency of the base circuit inductance combined with the emitter load capacitance happens to be within the range of frequencies where the input exhibits negative resistance, oscillation will occur. The base stopper resistor makes the net input resistance positive.
Other transistors which on the surface do not appear to be emitter followers, such as the transistors in a differential pair, can exhibit this phenomenon, especially if the tail current is provided by an active (transistor collector) current source.

Transistors with high Ft are especially prone to this problem.

#### Austin Clark

Joined Dec 28, 2011
412
do I need a resistor in ??? below?

Rich (BB code):
           vcc
|
resistor
|
|---------------???----------------transistor base
/
input----resistor----|  npn
\
|
gnd
I deal with these types of circuits a LOT. And no, a resistor there isn't necessary. However, if you were wanting to output to several transistors, you would need a resistor before each.

#### WBahn

Joined Mar 31, 2012
25,911
do I need a resistor in ??? below?

Rich (BB code):
           vcc
|
resistor
|
|---------------???----------------transistor base
/
input----resistor----|  npn
\
|
gnd
It depends on the particulars of the following circuit. For instance, if the following stage is an emitter follower, then you probably do not want a base resistor since you want the emitter voltage to track the input voltage to that stage, which is the collector voltage shown in your figure. But if you have a common-emitter circuit, then the emitter voltage of the follow on transistor might be tied directly to a fixed supply voltage, in which case you need to let the base voltage remain constrained to about a diode drop above it via a base resistor.

While not the only consideration, a good first answer can be determined by asking, under normal operation, if the emitter of the follow on transistor can/should remain at about the same voltage or whether it can more-or-less track the collector voltage of the prior transistor. An equivalent approach is to consider whether large voltage swings on the base of the follow on transistor will result in large base-emitter voltage drops in that transistor. If so, then you need to isolate it with something such as a base resistor.

Notice that the above discussion is focussed on bias currents. Even if you don't need a resistor from a DC (bias) point of view, you might need a resistor from an AC point of view, either for impedance matching or for stability. If you need a resistor from EITHER point of view, you need a resistor.

#### Ron H

Joined Apr 14, 2005
7,014
Base stopper resistors (I had never heard the term, but I have used them many times) are not required for stability in saturating stages, such as that posted by tpny. They are typically needed in emitter followers which have a small amount of cap loading on the emitter, and some possible parasitic inductance in the base circuit. The base of an emitter follower sometimes exhibits negative resistance at high frequencies. If the resonant frequency of the base circuit inductance combined with the emitter load capacitance happens to be within the range of frequencies where the input exhibits negative resistance, oscillation will occur. The base stopper resistor makes the net input resistance positive.
Other transistors which on the surface do not appear to be emitter followers, such as the transistors in a differential pair, can exhibit this phenomenon, especially if the tail current is provided by an active (transistor collector) current source.

Transistors with high Ft are especially prone to this problem.
I made a rash assumption that tpny's 2nd stage was to be a saturating NPN. WBahn made me realize that this wasn't necessarily so.
Attached are my rules-of-thumb, which I had never actually drawn out before.
There may be others that I didn't think of.
There are also cases where there may be an emitter resistor, but it is so small that a base resistor is still required for current limiting.
Also, the output may actually be taken from the collector in some circuits with emitter resistors.

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#### tpny

Joined May 6, 2012
216
yes, i was thinking of 2nd stage transistor as a switch. what size resistor do u need at the base (assuming one was necessary)?

What if one wasn't necessary but the collector resistor from the previous stage was too tiny causing too much current flowing thru 2nd transistor's base?

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#### Ron H

Joined Apr 14, 2005
7,014
yes, i was thinking of 2nd stage transistor as a switch. what if the collector resistor in the 1st stage is tiny? (eg, 200mA of current going into 2nd transistor's base..)
The minimum base current is ≈Ic/10. More is wasted. Less and you risk incomplete saturation with some transistors in a given lot.
The only purpose I can think of for that type of circuit is as a noninverting level translator. Why would you ever switch 200mA into the base of a transistor?

#### tpny

Joined May 6, 2012
216
well the circuit is fed off of a current loop containing about 20mA, i need the 1st stage transistor to exhaust about 18mA (by assigning properly sized collector resistor), but i need to also use that collector level to switch the 2nd transistor. (so i exaggerated by 200mA, but what about 18mA into the 2nd transistor's base..i would think i should add a resistor in fron of it to limit the current thru it..)

A different question: suppose a resistor is necessary (feeding collector level to 2 transistor switches at following stage for example), what should the size of base resistor be (relative to the collector resistor prior to it)?

#### Ron H

Joined Apr 14, 2005
7,014
well the circuit is fed off of a current loop containing about 20mA, i need the 1st stage transistor to exhaust about 18mA (by assigning properly sized collector resistor), but i need to also use that collector level to switch the 2nd transistor. (so i exaggerated by 200mA, but what about 18mA into the 2nd transistor's base..i would think i should add a resistor in fron of it to limit the current thru it..)
Can you post a schematic or block diagram of the current loop and the transistors?

A different question: suppose a resistor is necessary (feeding collector level to 2 transistor switches at following stage for example), what should the size of base resistor be (relative to the collector resistor prior to it)?
The base resistors should be sized so that Ic/Ib≈10 in each of the 2nd stage transistors. That's assuming you are not also switching something like an LED with the 1st transistor. If that's the case, then more details would be required.

#### tpny

Joined May 6, 2012
216
That's assuming you are not also switching something like an LED with the 1st transistor. If that's the case, then more details would be required.
what difference would the led make? thanks!

Also, another perspective, if the transistor's base is driven by 2 inputs (for example in a flip flop) do i need base resistor for both inputs, just one, none? Thanks!

#### Ron H

Joined Apr 14, 2005
7,014
what difference would the led make? thanks!
If the first transistor is switching an LED in the collector circuit, the base current into the second transistor(s) will flow through the LED when the first transistor is off, resulting in the LED still glowing when it should be dark. There are ways around this, depending on the application.

Also, another perspective, if the transistor's base is driven by 2 inputs (for example in a flip flop) do i need base resistor for both inputs, just one, none? Thanks!
You need to show a schematic for this. I'm not clear on what you mean.

#### Austin Clark

Joined Dec 28, 2011
412
Also, another perspective, if the transistor's base is driven by 2 inputs (for example in a flip flop) do i need base resistor for both inputs, just one, none? Thanks!
Well, the problem with connecting two outputs together directly like that is that, in some circumstances, if one output was HIGH and the other LOW, current would flow from the HIGH to the LOW, resulting in wasted power and an invalid voltage level. In other circumstances, like yours, you'll run into a situation where, if the outputs were connected, if either of them were to go HIGH, you'd essentially force the other output to go HIGH as well, so you couldn't use that output anywhere else if you wanted to.

So basically, don't do it. Make yourself an OR gate instead. You may be able to get away with using diodes on both outputs as well, but be aware of the voltage drop they'll cause.