A common way to do a gain stage is to combine a common cathode circuit with a cathode follower. The common cathode block may be a triode strapped pentode. Something like this:
[IMGDEAD]http://www.veiset.net/tech/P01.gif[/IMGDEAD]
Since the cathode of the CF is close to the plate voltage of the pentode it is temping to connect the screen grid to the CF’s cathode. This will result in feedback from the CF to the input stage.
[IMGDEAD]http://www.veiset.net/tech/P02.gif[/IMGDEAD]
This will at least do two things: reduce the output impedance and linearize the CF.
Here are some measurements of the two circuits. Blue curve is without and red curve with g2-feedback. The output impedance is 690 ohm for the plain CF and 280 ohm with g2-feedback.
Most of the distortion belongs to the common cathode block, so it might be worthwhile to play with the working point for that stage.
[IMGDEAD]http://www.veiset.net/tech/THD_g2_feedback.gif[/IMGDEAD]
Jan E Veiset
[IMGDEAD]http://www.veiset.net/tech/P01.gif[/IMGDEAD]
Since the cathode of the CF is close to the plate voltage of the pentode it is temping to connect the screen grid to the CF’s cathode. This will result in feedback from the CF to the input stage.
[IMGDEAD]http://www.veiset.net/tech/P02.gif[/IMGDEAD]
This will at least do two things: reduce the output impedance and linearize the CF.
Here are some measurements of the two circuits. Blue curve is without and red curve with g2-feedback. The output impedance is 690 ohm for the plain CF and 280 ohm with g2-feedback.
Most of the distortion belongs to the common cathode block, so it might be worthwhile to play with the working point for that stage.
[IMGDEAD]http://www.veiset.net/tech/THD_g2_feedback.gif[/IMGDEAD]
Jan E Veiset