# Vin vs. VB charts

Discussion in 'General Electronics Chat' started by hobbyist, Nov 6, 2009.

1. ### hobbyist Thread Starter Distinguished Member

Aug 10, 2008
774
62
Hi,
Here's a quick glance chart I made,
for determining the proper VCC, or Av. values with reference to the Vsig.in pk.

I've found that the VB. should be at least double the value of the peek amplitude of the signal voltage, when input impedance is high enough as to not have a distortion effect on the input signal.

And considering all output impedances when biasing the amp for the proper gain required.

Using these equations, Note: Ro = RC // RLoad

1. VCC / 2*Ro = Ic. (where Ic is the dc bias and signal currents)
2. Av = Ro / RE
3. Ic * RE = Ve
4. Ve + Vbe = Vb. (where dc bias VB is considered in the graphs to follow)
and rearanging and substituting gives this final equation:

VB = { ( VCC / 2 * Av. ) + Vbe} (where Vbe is 0.7v., and VB is the bias voltage on the base of CE amp.)

Example of how this could be used.

Say the pk. signal in is around 1v. pk.
and Av. is given as 10, so by using these charts I can approximate the VCC value needed to have this satisfied.

First I want VB to be double the pk. signal value at the input, so I want around 2v. at the base (VB).

So a quick glance at the chart shows for VB = 2V. and Av. = 10 would be a VCC of around 24 - 27 Volts.

On the other hand what if the power supply VCC is constrained to 12V.

and the pk. sig.input is 700mV, then I would have to use double that for VB which is 1.4V.

again a quick glance at the 12V. chart and a VB of around 1.4V. shows that the only voltage gain that can be achieved would be in the range between 5 and 10 .

Simple but effective.

Here is the charts.