Quote:
regenfreak: I am a bit confused about your comments that the differential configuration has lower input capacitance than the cascode configuration? Probably the explanation is very complicated. I read some theories on BJF amplifier design analyses..nothing is simple.
I thought the whole point of having cascode is to reduce the feedback capacitance, setting the gain for the lower common emitter BJT about 1. The upper common base BJT is not subject to Miller effect due to the grounded base shielding the collector signal from feedback to the input. Now the input capacitance of the cascode is the capacitance across the lower the B-E...but than the story is more than that.
|
Sadly, the cascode configuration will still show some increased capacitance at its input. This can be demonstrated via simulation of a cascode circuit using discrete BJTs. It's also possible to see the difference by looking at the extended datasheet for the CA3053 as it gives plots of input admittance in both cascode and differential configurations.
The b curve (susceptance) for cascode shows just over 1.5mmho at about 10MHz. This can be converted across to an equivalent parallel capacitance at 10MHz. This online calculator shows that this would be about 24pF
https://www.everythingrf.com/rf-calc...cra-calculator
In differential mode it is about 0.5mmho on the datasheet. This would be about 8pF. This is slightly higher than I was expecting but still much lower than the capacitance in cascode mode.
What isn't clear from the datasheet is how the cascode circuit is configured. I'd expect to see a bypass cap at pin 4 but the value of this cap will affect the b11 and g11 curves in the datasheet plot below. The g11 of 0.6 at 10MHz seems a bit lower than I expected especially if the bias resistors are factored in. Maybe this plot was taken with a very small value of bypass cap more suited to 100MHz operation. Otherwise, I'd have expected to see g11 at about 1 to 1.5 at 10MHz in cascode mode with something like a 10nF bypass cap.
The other minor niggle with operating the CA3053 in cascode mode is that it won't deliver symmetrical limiting with large signals. Only one side of the waveform will show flat topping. The differential configuration should give fairly symmetrical limiting so it ought to produce a classic square wave output when driven hard. This is probably better for FM reception but I don't know how significant this would be in reality.