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.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".
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.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.do I need a resistor in ??? below?
Rich (BB code):vcc | resistor | |---------------???----------------transistor base / input----resistor----| npn \ | gnd
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.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.
The minimum base current is ≈Ic/10. More is wasted. Less and you risk incomplete saturation with some transistors in a given lot.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..)
Can you post a schematic or block diagram of the current loop and the transistors?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..)
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.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)?
what difference would the led make? thanks!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.
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.what difference would the led make? thanks!
You need to show a schematic for this. I'm not clear on what you mean.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.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!