# Load line of transistor question

1. Nov 20, 2012

### Outrageous

In order to draw the load line of transistor, we have to find out the saturation current (where the voltage is minimum or zero) and voltage of collector-base when collector current is zero.

In order to calculate maximum saturation current, we have to forward biased the base-emitter, and reverse biased the base -collector and then get the voltage of collector-emitter to be zero(V of collector minus voltage of emitter equals to zero), right??

Thank you

2. Nov 20, 2012

### Outrageous

I think 'we have to forward biased the base-emitter, and reverse biased the base' instead of both forward because when we forward both we can't get a constant curve at saturation region, correct?

3. Nov 20, 2012

### rbj

the base-emitter junction is always forward biased as long as the transistor is not in full "cut-off" (where the collector current is zero). so, from the POV of the load line, the base-emitter is forward biased, usually by about 0.6 volts.

4. Nov 21, 2012

### Outrageous

what is POV?
The base-emitter is usually 0.7V.
How to draw a load line?

5. Nov 21, 2012

### Staff: Mentor

POV = "point of view"

6. Nov 21, 2012

### Staff: Mentor

Maybe this will help:

.

Last edited by a moderator: Sep 25, 2014
7. Nov 21, 2012

### Studiot

First of all there is not one load line for a transistor.

There are many and they depend upon the circuit configuration.

Having said that, for a resistive load the load line is a straight line so you only need to find two points and join them to be able to draw one.

To draw a load line for R ohms on the collector current (y axis) v collector voltage (x axis) curves:

1)Locate the point of the voltage axis corresponding to the supply voltage.

2)Locate the point on the current axis which the supply voltage would drive through R ohms.

3) Join these two points to form the load line for R ohms.

go well

8. Nov 21, 2012

### Outrageous

Last edited by a moderator: Sep 25, 2014
9. Nov 22, 2012

### rbj

okay. whatever. from long ago, when i was a graduate teaching assistant (and i was teaching a lab), it was most often 0.6v for silicon.

well, your transistor curve is that of a function $I_c$ as a function of $V_{ce}$ for various $I_b$. so given an $I_b$, you still don't know what $I_c$ is without also knowing $V_{ce}$. the load line is another equation that relates $I_c$ and $V_{ce}$. and that equation comes from the rest of the circuit (assuming no $R_e$:

$$V_{cc} = V_{ce} + I_c R_c$$

or

$$I_c = \frac{1}{R_c} ( V_{cc} - V_{ce} )$$

plotting that equation on top of the transistor curves is your load line.

10. Nov 22, 2012

### Outrageous

Understand.
when there is $R_e$ ,we can use the equation in this (transistor load line)