Rikard,

For a (very) rough check,
Tie a 10mA source to the collector of the bipolar. Increase the the
base current until the collector voltage drops to less than 0.6 (or
even just registers "low" on a pic input.) The ratio of 10mA to the
base current will give you the approximate hfe. at 10mA. (or 1mA or
10A etc.) Since the base current will be low, and the base voltage
will be in the 0.5 - 0.7 V area, a pwm generated voltage feeding a 10k
resistor to the base may be enough.

(50% pwm = 2.5V,
2.5V - 0.5V = 2V,
 2/10e3 = 200uA Base Current)

hfe = 10mA/200e-6 = 50.



RP
On 01/05/07, Harold Hallikainen <harold@hallikainen.org> wrote:
> Well, for your collector current sampler, you could do something like this:
>
> 1. Put a resistor from the output of an op amp to the inverting input, say
> 1k.
> 2. Drive the non-inverting input with the desired collector voltage, say
> +10V.
> 3. Connect the collector of the transistor to the non-inverting input of
> the op amp.
> 4. Ideally, the inverting input is at the same voltage as the
> non-inverting voltage, so it's at 10V. Ideally no current flows into
> either input of the op-amp, so all the collector current must flow through
> the resistor. The output voltage will then be 10V + Ic*1k. You can measure
> that output voltage and calculate the collector current.
>
> Harold
>
>
>
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