Sorry, I don't get the idea. 4 MHz isn't actually a high frequency, still suitable for wideband amplifiers. (I'm operating wideband circuits up to several 100 MHz)Capacitive matching should be taken into consideration between the two stages for such high frequency circuit.
Don't work isn't actually a clear observation. Of course, a DC path to the +ve input must be provided, according to the 2 uA bias current. If you possibly observe oscillations without the 50 ohm resistors, this would indicate mainly that the layout is unsuitable for a higher gain. In this case, the circuit may not work without the gain reduction provided by the resistors. Also unwanted feedback from amplifier output to the antenna may be an issue.The 2 stages together don't work without it.. I don't know why
Don't work isn't actually a clear observation. Of course, a DC path to the +ve input must be provided, according to the 2 uA bias current. If you possibly observe oscillations without the 50 ohm resistors, this would indicate mainly that the layout is unsuitable for a higher gain. In this case, the circuit may not work without the gain reduction provided by the resistors. Also unwanted feedback from amplifier output to the antenna may be an issue.
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So, looking in the output impedance graph in the datasheet of lt1364 I've seen that it's around 50 ohm (hard to see the exact value in a logarithmic graph). And so I've put the 50 Ohm resistor...
I've done this right or not?
thanks
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At the resonant frequency (4MHz), if the trimmer capacitor may aid to match the resonance of the circuit and provide better amplification.
OP (open loop) output impedance surely isn't a subject to impedance matching. In a reasonable designed OP circuit, the closed loop output impedance would be much lower than the open loop value, but in your circuit, there's effectively no loop gain left at 4 MHz. Thus the 50 ohm load will reduce the gain of the driving stage. At the input of the first stage, the 50 ohm load will completely cancel the effect of the tuned antenna, reducing the circuit Q below 1. I can't imagine, that it's wanted this way. Designing the first stage as a high impedance buffer seems the only reasonable option with a tuned antenna.So, looking in the output impedance graph in the datasheet of lt1364 I've seen that it's around 50 ohm (hard to see the exact value in a logarithmic graph). And so I've put the 50 Ohm resistor...
I've done this right or not?
I assume, the amplifier chain is oscillating due to uncontrolled feedback of your bredboard design. Even with a good PCB, it may be advisable to use two single OPs instead of a dual type.the 4 MHz component is lost in a very bad spectrum.
I don't hear substantial knowledge of OP circuits from your words.The amplifier individually working, but while cascading it fails means it must be an impedance mismatching as already told.
It would, with sufficient loop gain. But with 70 MHz GBW there isn't any in this circuit at 4 MHz.The output impedance of the LT1364 (about 50 ohms) will be drastically reduced due to negative feedback to a value below 1 ohm.
That's not a problem as such. Individual bypass capacitors and optional filter inductors in circuits with very high gain can effectively cancel supply crosstalk/feedback.I've forgotten to say that even if I use 2 separated opamp (2 separated chips) I'm using the same supply...
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