# Transistor biasing

1. Aug 2, 2014

### ranju

I have studied that "zero signal collector current should be atleast equal to the maximum collector current..." but what is the reason behind this?? and why we need to to consider this zero signal collector current .ultimately what is the significance..if we have to see when we apply the signal to the transistor..???

2. Aug 2, 2014

### Staff: Mentor

I doubt that you have heard that! It sounds wrong. This is for a Class A amplifier, is it?

Can you put this in context? Precisely which circuit are you referring to?

3. Aug 2, 2014

### ranju

No its the same which I read from the book...and I have no idea about the class of amplifier.. we have'nt studied this in much detail..
this proper zero collector current point is basic condition for faithful amplification of signal

4. Aug 2, 2014

### MrSparkle

what happens to small signals when they encounter the 0.7 or so voltage drop of transistor?

5. Aug 3, 2014

### Staff: Mentor

You textbook is talking about setting the zero-input collector current point .... which means the collector current when there is no input signal. Suppose you wanted an AC sinusoid output of magnitude 100mA peak. You would need to bias the collector current at a DC value slightly greater, say 110mA, so that negative peaks could drop collector current to 10mA while positive peaks could drive it to 210mA, enabling linear ampification of the input.

Last edited: Aug 3, 2014
6. Aug 8, 2014

### Sumesh

the zero signal collector current is needed because even in the absence of the input signal the battery which is connected will be helpful for the output to be obtained

7. Aug 10, 2014

### sophiecentaur

This really doesn't make sense. What is this "maximum collector current"? It is not likely that a circuit would be biased to give maximum collector current with no signal input. The context must be very unusual, I think. We rely do need a circuit diagram, to resolve this, I think.

8. Aug 10, 2014

### AlephZero

As NascentOxygen said, it makes sense if it means

If it was exactly twice, the DC collector current will swing from 0 to twice the zero signal DC current.

But the current can't go down to zero without causing some distortion, which is why it says "at least" .

9. Aug 10, 2014

### TurtleMeister

Restated: The value of zero signal collector current should be at least equal to the maximum collector current due to signal alone. The meaning of "signal alone" is when the amplifier is operated without bias where only the positive or negative half cycle is amplified.

See talkingelectronics.com section 9.1.

10. Aug 11, 2014

### sophiecentaur

I am confused about the terminology here. We could all be talking at cross purposes unless a diagram and some proper definitions appear. What can "zero signal" mean, if not the current due to the input bias level? What has been written above, sort of implies and 'upside down' class B/C operation, which would be pretty inefficient, with saturation current flowing except when input signal (negative) peaks reduce it. Where have I gone wrong?

11. Aug 11, 2014

### TurtleMeister

I was confused also. Especially the phrase "due to signal alone". The "zero signal" means no AC signal input to the base of the transistor. The section I linked to in post #9 cleared it up for me. It's a learning exercise for the biasing of a class A transistor amplifier. Figure (i) shows a transistor without biasing (class B). The peak collector current in this mode (with AC input applied) would be the minimum "zero input (without AC input applied)" collector current for faithful amplification in class A mode (figure (ii)).

Of course there's more to learn with biasing transistors, but this was the particular aspect that the op was questioning.

12. Aug 12, 2014

### sophiecentaur

So the peak of Collector current should be about twice the "zero signal current", imo. (This is not what the OP says he/she has "studied".) This thread demonstrates, yet again, how valuable a diagram in the very first post can be. Newbies to the subject often think everyone knows what their question is about but it's often not the case and it can take many posts to find what they really want to known.
Then there may be linearity issues (sometimes but not always). For some classes of amplifier the "zero signal current" would be zero.
"That' link should be viewed with a bit of care. Figure 9.1 shows the unbiased amplifier as non-inverting. Not a good start, I think. I haven't looked anywhere else, so it may be just a single blip.

Last edited: Aug 12, 2014