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I'm trying to fully understand some tuned amplifier circuits and i guess this should be really simple to you guys.Here are them:

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I would like ask you three things things, each one regarded to each circuit :

Circuit 1 - Here i understand the upper transformer is meant to do a coupling to the final stage ( how does a transformer coupling help ? ) .Am i right that the final stage is a selective filter that is necessary to have so that in Vo we get only the frequency we want ? I also want to understand why do we need to put a capacitor in paralel with Vin before throwing the signal into the lower transformer.

Circuit 2 - I don't see clearly what L3 and C2 are doing, do they form a tank ( selective filter ) ? , why would we need a tank there ? The only thing i know C3 is meant to block the AC for the bias.

Circuit 3 - Okay, now in initial stage we got a input current, does it change anything? Also, in the final stage we have not a RLC circuit, but a RL filter ... Does that change anything or has any purpose ?

Thanks a lot in advance.

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  • \$\begingroup\$ Your first circuit is problematic in that it shows a voltage generator across C2 and this destroys the effect of the emitter tuning. If it were a current injection like (2) and (3) then no problem. \$\endgroup\$ – Andy aka May 15 '13 at 8:38
  • \$\begingroup\$ "and i guess this should be really simple to you". Well, thanks for putting so much trust in us. We're mostly dealing with people calculating LED current limiting resistors and not killing themselfs by touching mains wires here :-)... Anyway, nice question! \$\endgroup\$ – Nils Pipenbrinck May 19 '15 at 13:56
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Circuit 1 Questions:

i. The transformer acts as an impedance transformer 1:4

ii. The transformer secondary is a tank circuit acting as selectivity network (freq selective), most probably low Q to have good bandwidth response.

iii. Just like the output, the lower transformer have selective input before impedance is transformed to the transistor

Circuit 2 Questions:

i. L3 and C2 again is on freq selective input duty

ii. C3 is usually calculated to be reactively insignificant (short circuit) to the resonance freq.

Circuit 3 Questions:

i. Lower transformer is acting as freq selective network the top transformer acting as broadband power transformer (thus no freq selective properties).

ii. This configuration used in power amplifier modules commonly available in integrated packages. Post filtering is usually required.

iii. The output is untuned (only RL instead of RLC) as it usually utilizes a core (usually ferrite alloys) that can handle large power if untuned.

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