# From the Archives Archived discussions on CrazyEngineers

@thaiffer • 26 Nov, 2009
I need to determine the maximum voltage swing for a Transistor circuit I have constructed. I have researched it in books and online but I'm struggling to understand it. The circuit uses a BC108B transistor, I have recorded some of the values from this circuit, and have established that the load line runs from 3.03mA on the X axis to 10V on the Y axis of my load line operation chart. I have been told that my Q point is the same as my Vc value which I have recorded as 5.44V, the value of IC I have read off from my chart corresponds with my value of Ic which I recorded at 1.382mA.

I now need to calculate my voltage swing but I am not sure how to do so, if someone is able to help me that would be most useful. I can provide a lot more information/diagrams relating to this circuit if required but at the moment I'm not sure what further information is relevant.
@reachrkata • 27 Nov, 2009 Ok here goes.
I presume that you are requesting the output voltage swing calculation for the normal CE amplifier ckt with Rb = base resistor, Rc = collector resistor. Emitter is connected to GND -

To determine the output voltage swing exactly, you need to calculate range on the load lime where the transistor is in active region. Anything in saturation / cut-off means clipping at the output.

1) First you find the Ibmin required for the transitor to just go into active region from cutoff. With the Ibmin*Hfe will give Ic. From this Ic you can calculate the collector voltage value Vc1.

2) Next you find the the Ibmax required for the transitor to just go remain in active region without going into saturation. With the Ibmax*Hfe will give Ic. From this Ic you can calculate the collector voltage value Vc2.

Vc1 ~ Vc2 is the voltage swing you require.

- Karthik
@thaiffer • 03 Dec, 2009 Sorry does it make any difference that I have a collector resistor (3.3k ohm) and an emitter resistor (1k ohm).

I measured Ib min is 40nA and Ib max at 8.5uA, my hfe gain is 200 so i calculate that my low value of Ic is 8uA, my high value of Ic i calculate to be 1.7mA.

Reading off from my load line I approximate VC1 at 10V and VC2 at 3.5V.

Can you confirm this is the correct method?
@thaiffer • 03 Dec, 2009 I just checked these values of VC on my multisim circuit and can confirm that for VC1 I record 9.9V when the amplifier is just turned on, and VC2 when it just saturates I record at 3.3V.
@reachrkata • 03 Dec, 2009 Great !! Then the maximum voltage swing is 9.9 - 3.3 = 6.6V.

This behaviour can also be simulated in Multisim. To your circuit, just give a sine wave input at the base and check the sine wave output at the collector. Verify at the collector that the waveform starts clipping after a peak-peak of 6.6V.

-Karthik
😁

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