Common Emitter Active Load Diode Drop Causes Voltage Offset

[fobos3]

Hi,

I am trying to design a common emitter amplifier with a current mirror for active load for an AGC. The problem I am having is that the voltage at my base is 0.7V + Vin. This causes a voltage offset when using small gain and saturation when using large gain, which is specified by the feedback network. Any ideas on how to solve this?

Regards

P.S.
Gain between Vb and Vout is about 59dB

 

[erikl]

For different gain settings you'd probably need different bias concepts or application circuits.

In order to get more help, could you, possibly, show your current schematic?

 

[fobos3]

Here is my circuit:

 

[LvW]

Here is my circuit:
View attachment 84965

Looks fine - but what`s your problem?

 

[fobos3]

The problem I am having is that the output of the amplifier is given by:
\[V_{out} = V_{be} - \frac{R_1}{R_2}(V_{in} - V_{be})\]
If I set the gain to -1, I have a sine wave centred at 1.4V. If I set the gain to -2, the sine wave is centred at 2.1V etc. Eventually, if the gain is large enough, the output reaches the positive power rail and my my output is just DC.