Transistor questions

[Richardcavell]

1. For a bipolar junction transistor, is it always the case that the base-emitter junction encounters a voltage drop, while the collector-emitter never does?

2. Is there a type of transistor that works like a relay, with 4 pins, such that a current between two input pins will connect the two output pins? Or do all transistors have one leg in common?

3. If there is a base-emitter voltage sufficient to saturate the transistor, can the polarity of the collector-emitter current be reversed?

4. It troubles me to speak of a 'saturation voltage' when the collector-emitter current is still tied to be some constant of the base-emitter current. This means that to obtain a certain current as output, one might need an input current that is higher than the stated "saturation voltage." Am I right in thinking that?

Richard

 

[Grossel]

1. For a bipolar junction transistor, is it always the case that the base-emitter junction encounters a voltage drop, while the collector-emitter never does?

A transistor without a voltage drop doesn't work - is said to be broken.

2. Is there a type of transistor that works like a relay, with 4 pins, such that a current between two input pins will connect the two output pins? Or do all transistors have one leg in common?

I beleive I once a time heard of a type of cmos that had two gates. But I cannot recall if there was a relationship between voltage difference between the two gates.

3. If there is a base-emitter voltage sufficient to saturate the transistor, can the polarity of the collector-emitter current be reversed?

That would probably destroy the transistor. You may be able to use a jfet for that purpose.

4. It troubles me to speak of a 'saturation voltage' when the collector-emitter current is still tied to be some constant of the base-emitter current. This means that to obtain a certain current as output, one might need an input current that is higher than the stated "saturation voltage." Am I right in thinking that?

You're right about the relationship between Ib (base current) and Ic (collector current). Say you just have emitter grounded and a resistor is connected from collector to positive voltage supply.
What happens when you slowly increase Ib? The collector will draw a current that is Hfe times greater.
In theory, how much current (maximum) can the transistor draw through collector? That depends of supply voltage level and resistor value.
When you applies even more current to base, the Ic current cannot increase any more, and the transistor is said to be saturated. Then it is virtually no voltage drop between emitter and collector.

Hope this help

 

[BrownOut]

1. For a bipolar junction transistor, is it always the case that the base-emitter junction encounters a voltage drop, while the collector-emitter never does?

There is always a collector-emitter voltage drop.

2. Is there a type of transistor that works like a relay, with 4 pins, such that a current between two input pins will connect the two output pins? Or do all transistors have one leg in common?

I've never seen a transistor with a leg. All or nearly all transistors have 3 pins or leads, but they can still operate like a relay.

3. If there is a base-emitter voltage sufficient to saturate the transistor

That depends on other things.

can the polarity of the collector-emitter current be reversed?

No.

4. It troubles me to speak of a 'saturation voltage' when the collector-emitter current is still tied to be some constant of the base-emitter current. This means that to obtain a certain current as output, one might need an input current that is higher than the stated "saturation voltage." Am I right in thinking that?

That doesn't make sense. The amplitutes of current and voltage can't be compared. Like apples and oranges.

But this might help. A BJT is saturated when it's C-B junction becomes forward biased. Until that point, the collector curernt is IC = β*IB, and the collector voltage is VCC - IC*RC, thus saturation is:

VC <= VB - .6V for NPN, and

VCC - βIB*RC <= VB - .6V, for NPN.

Note: the symbol <= means "less than or equal to"

 

[crutschow]

1. For a bipolar junction transistor, is it always the case that the base-emitter junction encounters a voltage drop, while the collector-emitter never does?

2. Is there a type of transistor that works like a relay, with 4 pins, such that a current between two input pins will connect the two output pins? Or do all transistors have one leg in common?

3. If there is a base-emitter voltage sufficient to saturate the transistor, can the polarity of the collector-emitter current be reversed?

4. It troubles me to speak of a 'saturation voltage' when the collector-emitter current is still tied to be some constant of the base-emitter current. This means that to obtain a certain current as output, one might need an input current that is higher than the stated "saturation voltage." Am I right in thinking that?

At the risk of confusing you I will add my 2 cents worth.

1. The collector-emitter drop can be less (but still finite) when the transistor is saturated as compared to the base-emitter drop which is typically about 0.7V.

2. No. All transistors have a common connection between input and output. Only something like an opto isolator can provide isolation between input and output.

3. Yes within limits. You can interchange the emitter and collector, and the transistor will still work but generally at a greatly reduced beta (gain).

4. You seem to be confusing voltage and current. "an input current higher than the saturation voltage" makes no sense. The base-emitter voltage can be higher than the saturation voltage (see answer to 1.), but the base-emitter current can be less than the collector-emitter current for saturation to occur. Saturation just means that the transistor is fully turned on, with the saturation voltage being the collector-emitter voltage when the transistor is saturated.

 

[BrownOut]

Yeah, I guess the answer to #3 is "it's possible" but there would be no practical reason to do it.

 

[crutschow]

Yeah, I guess the answer to #3 is "it's possible" but there would be no practical reason to do it.

As I recall, in the early days before FETs were common for switching low level signals, bipolar transistors were used as analog multiplexer switches in the inverted mode, since the "on" offset voltage was less.