First let me ask this. I know beta changes per manufacturing and so on but lets just assume it doesn't for a second

Say we have a BJT that has a beta of 100 in linear

What cant we use our beta equations for circuit analysis when we are in saturation?

For example,
IC = Beta * IB

Why is that not valid in saturation mode?

 

First let me ask this. I know beta changes per manufacturing and so on but lets just assume it doesn't for a second

Say we have a BJT that has a beta of 100 in linear

What cant we use our beta equations for circuit analysis when we are in saturation?

For example,
IC = Beta * IB

Why is that not valid in saturation mode?

For the very reason you describe. Even if you assume that the beta is a fixed, constant, unchanging value of 100 in the linear region, when you are in saturation you are NOT in the linear region. In fact, in this mythical world, you could use that as the very definition of when you are in the linear region -- when the beta is 100. As soon as the beta falls below 100, you declare that you are no longer in the linear region and have entered the saturation region.

 

Some people have a tendency to think that if a process is linear somewhere it should be linear everywhere. Well the world doesn't work that way. If it did then if you increase base current without limit, you would increase collector current without limit as well. Physics tells us there are no processes that can increase without bound. Everything has a finite limit.

 

What cant we use our beta equations for circuit analysis when we are in saturation?

Look at the Vce voltage where they measure beta.
It's usually several volts, not the low saturation voltage.

 

In other words, you can't use Beta for a useful calculation when the transistor is saturated. Considering a grounded emitter inverter for example, that's because the load and power source set the collector current. Beta just has to be "enough".

 

First let me ask this. I know beta changes per manufacturing and so on but lets just assume it doesn't for a second
Say we have a BJT that has a beta of 100 in linear
What cant we use our beta equations for circuit analysis when we are in saturation?
For example,
IC = Beta * IB
Why is that not valid in saturation mode?

Don`t overstimate the role of beta in circuit design.
Two common emitter stages with the same operational DC point but different beta-values (e.g. 100 and 200) will have the same voltage gain. The different beta values influences the signal input resistance only.

 

Some people have a tendency to think that if a process is linear somewhere it should be linear everywhere. Well the world doesn't work that way. If it did then if you increase base current without limit, you would increase collector current without limit as well. Physics tells us there are no processes that can increase without bound. Everything has a finite limit.

So that makes sense but let me ask this

What your saying is, at some point, regardless of how much base current we draw the collector can only take so much.

At some point the DC gain tappers off.

so here Is my question.
In saturation the BJT call it simply is a closed switch with some small VCE .07V or something.

So why would we say that beta is very low in saturation. Wouldn't we say that beta is very high in saturation?

 

So why would we say that beta is very low in saturation. Wouldn't we say that beta is very high in saturation?

Once a transistor is saturated, further increases in base current don't result in increased collector current.

 

So that makes sense but let me ask this

What your saying is, at some point, regardless of how much base current we draw the collector can only take so much.

At some point the DC gain tappers off.

so here Is my question.
In saturation the BJT call it simply is a closed switch with some small VCE .07V or something.

So why would we say that beta is very low in saturation. Wouldn't we say that beta is very high in saturation?

For many applications, a BJT in saturation is treated as a closed switch. The voltage across the switch, Vce, is often treated as being 0 V. If that's too much of a simplification, then a value known as Vcesat is used and, for most silicon transistor applications, that is usually taken to be something around 200 mV.

The reason that we say that beta is very low in saturation is that, as you said, the DC gain (also known as beta) tapers off, meaning that it gets smaller. The more we go, the smaller it gets.

The reason we wouldn't say that beta is very high in saturation is because it isn't very high in saturation.

Keep in mind that "very low" and "very high" are qualitative terms and only have meaning in the context of a comparison to something else. The something else is the beta in the linear region. So what we are really saying is that, in saturation, the beta is very low relative to the beta in the linear region.

 

In saturation people often refer to a "forced" beta. A typical value to guarantee saturation for a transistor with a beta in the linear region of say 150 would be 10. To force the beta to be 10 guarantees that the transistor is in saturation with a low Vce(sat) of 200 mV.