On current measurement: If a resistor is available whose reactive impedance (in the relevant frequency band, XL = 2 * pi * f * L,  XC = 1 / (2 * pi * f * C)) is negligible compared to its resistance, you can use it as load or in series to the load, measure the voltage at this resistor (using an oscilloscope), and calculate the current from the result.  If this is not feasible, you had to use some kind of current clamp which measures the magnetic field caused by the current.  If your output signal contains frequency components below a certain threshold (e. g. 45 Hz) or even a DC component, you have to choose a current clamp capable of measuring DC currents.  You should bear in mind that both a resistor in series to the load as well as a current clamp introduce an impedance into your circuit, so the current during the measurement does not exactly equal the current flowing when the means of measurement are removed.

On current calculation: Generally, if the amplifier is used in the small signal domain, then the output behavior of the amplifier can be modeled by an ideal voltage source and an output impedance, and if the properties of the load are known, you can calculate the series circuit of output impedance and load, and apply Ohm's law to the complex voltage and impedance.  If the amplifier is driven to the limits of large signals (BJT nearly saturated resp. nearly switched off) you might get more realistic results if you consider these states and the transitions between them separately.

I hope this helps. A more detailed discussion could be based only on the schematics of amplifier and load, and knowledge of the parameters of the input (or output) signal.

On current measurement: If a resistor is available whose reactive impedance (in the relevant frequency band, XL = 2 * pi * f * L,  XC = 1 / (2 * pi * f * C)) is negligible compared to its resistance, you can use it as load or in series to the load, measure the voltage at this resistor (using an oscilloscope), and calculate the current from the result.  If this is not feasible, you had to use some kind of current clamp which measures the magnetic field caused by the current.  If your output signal contains frequency components below a certain threshold (e. g. 45 Hz) or even a DC component, you have to choose a current clamp capable of measuring DC currents.  You should bear in mind that both a resistor in series to the load as well as a current clamp introduce an impedance into your circuit, so the current during the measurement does not exactly equal the current flowing when the means of measurement are removed.

On current calculation: Generally, if the amplifier is used in the small signal domain, then the output behavior of the amplifier can be modeled by an ideal voltage source and an output impedance, and if the properties of the load are known, you can calculate the series circuit of output impedance and load, and apply Ohm's law to the complex voltage and impedance.  If the amplifier is driven to the limits of large signals (BJT nearly saturated resp. nearly switched off) you might get more realistic results if you consider these states and the transitions between them separately.

I hope this helps. A more detailed discussion could be based only on the schematics of amplifier and load, and knowledge of the parameters of the input (or output) signal.

This just nailed it. Thank you, Joerg.

Dear Sir,

For the BJT, forward (I assume you are not interested in the behaviour at saturation), the equation for the current is given by:

Ic = Io * exp(Vbe/Vt)

Ib = Io * exp(Vbe/Vt) / beta

Qb = tau-F * Ic

with

Io : Some constant, get it from the datasheet

Ic : the collector current

Ib : the base current

Qb : The charge collected in the Basis due to the transistor turning on.

        (This is the charge on Base-Emittor capacitor associated with the

          transistor conducting current)

Vbe : The base-emitter voltage

tau-f : the forward time constant (tau-f = 1/(2*PI*Ft), with Ft the transition frequency.)

beta : the current amplification factor of the transistor

Vt = kT/q with

  k : bolzmanns constant (1.38e-23)

  T : absolute temperature (293 K at room)

  q: the charge of the electron (1.6e-19)

You can of course linearize the model for small signal, but as given here, its both

valid for small signal and large signal.

Best Regards,

Henri.

Thanks, Henri. 

I think that Henri calculation is only theoretical because the beta factor can't be precisely determinated and the real value changes with current and temperature. If you want to find the current for a specific circuit you can apply Joerg method by using non inductive and non capacitive resistor.I think that the general method for finding currents in electronics is by measuring voltage at specific value resistor. The voltage can be measured with DC oscilloscope.

Mircea