Does supply voltage affect collector current in transistor?

Question

(Forgive me if I make a few mistakes in the text. My knowledge of the English language is poor.) I started learning how transistors work and I've got a question. Does the supply voltage affect the collector current? Since we have a resistor in the collector circuit, we should have a current in a collector circuit. I know that books say that base current affects collector current, but I haven't seen any of them mention the effect of supply voltage on it.
Edit: I guess I should specify that I'm talking about common emitter amplifier

Answer 1

If base current \$I_B\$ is held constant, then collector current \$I_C\$ will obey this relationship:

$$ I_C = \beta I_B $$

\$\beta\$ is the transistor's DC current gain. This will be approximately true regardless of conditions at the collector. In the circuit below I keep \$I_B\$ constant at 1mA, and I vary the supply voltage \$V_S\$:

^{simulate this circuit – Schematic created using CircuitLab}

This is a plot of collector current \$I_C\$ as I sweep supply voltage \$V_S\$ from 4V to 16V:

As you can see, \$I_C\$ remains fairly constant at 150mA throughout, indicating that this transistor has current gain \$\beta=150\$. There is some variation of \$\beta\$ as the potential difference \$V_{CE}\$ between collector and emitter changes, but it is mostly assumed to remain constant.

This invariance of \$I_C\$ will not be true if changing supply voltage causes a change in base current. If, for instance, I use a resistance to provide base current from the supply (in contrast to the 1mA constant current source I1 above), this will obviously make \$I_B\$ highly dependent on the supply:

^{simulate this circuit}

Clearly \$I_B\$ here will increase as supply voltage \$V_S\$ increases, and we will see a commensurate change in collector current \$I_C\$, plotted here against \$V_S\$:

However, this varying \$I_C\$ is a consequence of changing \$I_B\$, and is only illustrating how you should be careful when you use phrases like "varying supply voltage". Are you really referring to "supply voltage", or do you mean conditions at the collector (\$V_{CE}\$) alone?

Answer 2

Google pictures of curve tracer output for a transistor. Note that Vce tends to be on axis X, and the resulting Ic on axis Y. The diagram typically shows several traces, at specific levels of base current Ib.

There's a simple transistor-based circuit that strives specifically to have the collector current independent of collector voltage - it's called the constant current source, and it's a crucial building block of modern linear analog electronics and integrated circuits. As a further reading, even just to take a glimpse ahead if you're a novice, I'd suggest a free book called Designing analog chips by Hans Camenzind.

I agree with the others - supply voltage is a parameter of the whole circuit topology, not just the sole transistor. So I do not mean to put Vce equal to supply voltage - they may be two different things.

One tangential note: as a general rule, increasing the supply voltage of a circuit tends to make it work faster. Logic achieves faster switching, analog amps get a higher slew rate and broader bandwidth. An analog amp that's borderline unstable may be convergent below a certain level of power supply voltage, but may oscillate at higher power supply voltages.

Answer 3

This question is conceptual, and to answer it, you do not need to know the internals of the transistor, but rather its behavior.

In the common-emitter amplifier stage, think of the transistor collector-emitter part as a "dynamic resistor" with varying static resistance Rce = Vce/Ic. A collector resistor with constant resistance Rc is connected in series to the "dynamic resistor" Rce. So the collector current is determined by the supply voltage Vcc and the total resistance Rtot = Rc + Rce, at first glance, according to Ohm's law - Ic = Vcc/Rtot.

If Rce was an ordinary static resistance, when Vcc increases, the current will also increase according to Ohm's law. But here Rce does not stay constant, and also increases so that to keep a constant ratio Vcc/Rtot = Ic.

So the transistor acts as a current stabilizer with adjustable resistance, and (ideally) the supply voltage does not affect the collector current.