# Estimating Vc of transistor (darlington)

1. Jun 14, 2010

### TsAmE

1. The problem statement, all variables and given/known data

Estimate the collector voltage VC when vin is +1, +2 and +3V. (assume IC/IB = 1000)

2. Relevant equations

IC/IB = 1000

3. The attempt at a solution

I know that a transistor becomes saturated when VCE and RCE are reduced to 0 and when the maximum IB flows, but in this instance, how can you tell if the darlington is partly-on or fully on (for the +2 and 3V)?

#### Attached Files:

• ###### Darlington.png
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2. Jun 14, 2010

### Staff: Mentor

That's not a Darlington connection. But the gain you quote (1000) would fit with a Darlington connection.

Is the figure misdrawn? It's important to show the full (2 x NPN) Darlington configuration if you want to get the voltage drops correct.

3. Jun 14, 2010

### TsAmE

Sorry I ment for an NPN transistor, there was a similar question with a darlington I did thats why I wrote darlington by mistake.

4. Jun 14, 2010

### vk6kro

With one base-emitter junction, assume 0.6 volts across it before current will flow.

So, work out the voltage across the 200 K resistor for the various input voltages (ie subtract 0.6 volts from each input).

Then work out the current in the 200 K. This is the base current.

Then multiply this by 1000 to get the collector current.

Then work out the voltage across the collector resistor when that current flows in it. If this is more than the supply voltage, the transistor is saturated and this current can't really flow.

Subtract this from the supply voltage to get the voltage across the transistor.

Last edited: Jun 14, 2010
5. Jun 15, 2010

### TsAmE

Oh I see, thanks a lot :)