Colin hoof
Member
In the process of constructing a wideband home brew rf signal generator,I noticed some very strange shinanagins with my pp3 9 volt
Battery connector.A simple colpitts oscillator followed by a buffer stage was constructed ugly style on a copper clad board.The current drawn by the buffer circuit was observed.In trying to match impedance of my next stage which will form a cascode amplifier.The emitter resistance of the buffer stage was varied to see how close I could get to the input impedance of the cascode stage ( common base amplifier which could be as low as 30 ohms.Without the buffer the colpitts oscillator signal has an output impedance of about 2.6k ohms.Definately too high to be of any use in a cascode circuit.Most commercial rf generators have an output impedance of 50 ohms which is what I was trying to achieve.As I lowered the emitter impedance down , I noticed the oscillator signal gave up at around 500 ohms.When I pressed hard against the pp3 battery connector I noticed the signal was restored,I could then lower the emitter resistance furthur until I achieved my 30 ohm target and drew around 30 ma of collector current in the buffer stage.Can anyone explain this weird behaviour as I have never noticed this before.
I was trying to keep the supply to the rf circuit as clean as possible,by utilising a battery supply instead of a mains psu.Also plenty of decoupling,rf choke use and good construction techniques,keeping all lead lengths as short as possible.
Battery connector.A simple colpitts oscillator followed by a buffer stage was constructed ugly style on a copper clad board.The current drawn by the buffer circuit was observed.In trying to match impedance of my next stage which will form a cascode amplifier.The emitter resistance of the buffer stage was varied to see how close I could get to the input impedance of the cascode stage ( common base amplifier which could be as low as 30 ohms.Without the buffer the colpitts oscillator signal has an output impedance of about 2.6k ohms.Definately too high to be of any use in a cascode circuit.Most commercial rf generators have an output impedance of 50 ohms which is what I was trying to achieve.As I lowered the emitter impedance down , I noticed the oscillator signal gave up at around 500 ohms.When I pressed hard against the pp3 battery connector I noticed the signal was restored,I could then lower the emitter resistance furthur until I achieved my 30 ohm target and drew around 30 ma of collector current in the buffer stage.Can anyone explain this weird behaviour as I have never noticed this before.
I was trying to keep the supply to the rf circuit as clean as possible,by utilising a battery supply instead of a mains psu.Also plenty of decoupling,rf choke use and good construction techniques,keeping all lead lengths as short as possible.