TV modulator/amplifier connection

[neazoi]

Hello I would like to combine these two circuits to form a small transfmitter.
The output of the modulator is through a 2.7pf and the input of the amplifier is through a 22pf.
Should i remove one of the two capacitors and couple the two circuits with just one capacitor?
Which one should I remove, the 22pF or the 2.7pF
I guess for stability reasons and for lightly loading the modulator, I should keep just the 2.7pF?

 

[chuckey]

I would keep the 2.7 PF, a 22PF in series will not materially change its effective value.
Frank

 

[neazoi]

I would keep the 2.7 PF, a 22PF in series will not materially change its effective value.
Frank

Thanks,
I was also wondering if I can replace the BFY90 on the amplifier section, with 2sc9018.
By looking at the datasheets they have similar characteristics. Can I do it, or what changes would be needed to replace them?

 

[BigBoss]

Thanks,
I was also wondering if I can replace the BFY90 on the amplifier section, with 2sc9018.
By looking at the datasheets they have similar characteristics. Can I do it, or what changes would be needed to replace them?

Transistors look replacable,no issue..
2.7pF has been used to drive an antenna to prevent the oscillator stopping.Because if oscillator is loaded with relatively low impedance it may stop to oscillation.
However, the capacitor at the input of the amlifier (22pF) is used because it's been supposed that a 50Ohm(75Ohm) generator will be used.That's why neither 2.7pF nor 22pF may not be optimum value.I reecommend you to choose an optimum capacitor value for this coupling by cut-and-try method.
2.7pF may be too small to drive your amplifier so output level can not reach to the wanted level.In the other hand, 22pF may be too large to eventually to stop the oscillation.
Take this into your considerations.

 

[vfone]

The best option is to use a simple Diplexer made by a LPF and HPF, providing in this way some isolation between transmitters, otherwise you get different coupling problems.
Increasing the number of poles of the filters, you get better isolation.