ac and dc load line... BC547B

Is this a homework assignment? Most folks no longer use load lines in their routine BJT design work, since Ebers-Moll and the other semiconductor parameters are better for predicting how a circuit will work. I have BC547 datasheets from five different manufacturers in my computer, and although they have lots of specs and curves, none of them shows the transistor's characteristic curves. The General Semi and Siliconix / Vishay datasheets have plots of the variation of the h-parameters with collector current, but nobody uses that anymore either. Using the datasheet info, one could draw up a set of characteristic curves, would learning how to do that be helpful?

Hello Win,

So, if someone is still using Smith charts does that mean this person is considered "old"? (I hope not...)

Regards, Joerg

no im not old, its my professor's who's old.... he wants to see all these stuff in our project.

yes please

Sometimes the old stuff isn't very useful. We also had that at my university. I just learned it for the respective exams and then pulled the "flush" handle in my brain. Other old things are very useful, such as Smith charts.

Regards, Joerg

??? You're welcome.

Cheers! Rich

-- Elect Me President in 2008! I will: A. Fire the IRS, and abolish the income tax B. Legalize drugs C. Stand down all military actions by the US that don't involve actual military aggression against US territory D. Declare World Peace I.

Do you think that what they are asking for might be the characteristic of the circuit in which the transistor is operating rather than the properties of the transistor itself?

I suspect that.

If you imagine various different DC collector currents in the transistor and plot the collector voltage as a function of the collector current, then that might be what they want for the DC load line.

If you calculate the normal quiescent voltage of all of the nodes in your circuit, then replace all of the capacitors with fixed voltage sources which are the same as the voltages across the capacitors that you replaced (i.e. freeze the voltage across the capacitors), and if there are any inductors then replace these with current sources set equal to the current that was flowing in the inductor, and then try the above exercise of plotting collector voltage for various collector currents with your newly mangled circuit then that's possibly what they want for the AC load line, I would expect.

Chris

That's just a straight line from Vcc on the Vce axis to Vcc/Rc on the Ic axis. Then you fix the operating point at the intersection with the Ic vs Vce transistor output characteristic of the transistor at a fixed Ib, which can be obtained with SPICE by sweeping the collector supply from 0 to Vcc while holding base current at Ib using current source and plotting the resulting Ic. The AC load line is just a line of slope

-1/Rac drawn through the quiescent point, where Rac is the small signal ac-equivalent load resistance.

That's good, but it was the easy part. Now help him plot some characteristic curves for the BC547B to place the load lines on.

What was that all about?

I was just paging through new messages, and here's an entirely context-free one, so thought I'd respond in kind. ;-)

Cheers! Rich

He should use SPICE for that, plot it, and manually draw his load lines.

OK, not what I had in mind, but certainly the best way to quickly get a fairly-accurate graph of characteristic curves. And changes can be made to the transistor's spice-model parameters to quickly plot additional sets of curves covering the range of variations one would be likely to see from the transistor manufacturer. Not that we should be encouraging the practice of load-line plotting for transistors; that's a technique more appropriate to tube design in the old days. With Ebers-Moll, Early and Gummel-Poon to help us, there are better ways now to approach transistor-amplifier design.