Monday, February 2, 2015

Small Signal Amplifier, Buffer Amplifier

On many occasion new friends ask me this....I need some sort of amplifier which does not oscillate, it shall be easy to tune, should accommodate small as well as big signal , low noice. It has be used in the front end of the DC receiver. I need to drive a linear, some measuring instruments,i also don't have & not able to finding RF transistors etc.

Asking for all these parameters /constraints it is  like finding a perfect woman on first date. But, anyway here we make some trade-off and it works out. The small signal  amplification  in TX or RX needs more care. To arrive at first 1 watt power is little harder. After that its relatively easy to achieve 10 to 50watt with careful choice of the linear devices and Vcc.

Small Signal RFAmplifier:-

This RF amp is very wide. Important is that you choose the transistor wich can dissipate upto about 30ma dc current without any problem i.e metal pack So 2N3866 fits the bill perfectly. another suitable candidate is 2n5109.

Limit the current to 10ma and garden variety BC549c can do the job for you. In bc547/8/9 variety this transistor has highest gain. Another candidates you should look for are in C series C945 etc C2570 but it should be able to sink this current.

"ft, beta/gain, current, this  three parameters."

Balun core or toroid is a must. I have not used plastic former, but you can give it a try ! In the fig1 at R2. For transmitters don't put any resistors. For receiver, to limit the current use 100 ohms. ...
This buffer amp can be used after the oscilator. It gives good isolation. In the BITX version 3 its used as receivers IF stage. Its input resitance depends on the  input resistors value , say from 220 ohm to 1k ohm. Here transistors used are, first BF194b and second BC547b. Vcc can be from 5 volt to 12 volt. D is 1n4007 diode. Output impedance depends on the emiiter followers resistance i.e 470 ohm.

After this buffer amp you can use the above RF amp to drive a (100mw) linear, DBM, mixer stage etc . This is my standard buffer stage to be used in all receiver/ transmitter oscillator.

For Transmitter use:- Remember its very wide. It will amplify equally signal and harmonics. Use tuned amplifier with filter for further tx use.After this stage some sort of filtering is needed if your source is harmonics rich i.e oscillator.
buffer amplifier

How  to wind the bifiliar transformer. have a look here. Also see previous post in this blog.

(tuned amplifier and filter stage) these things I am using frequently in my design....
Small Signal Multiple Feedback Amplifier

Details at the http::/
Second amp is used in JOTA receiver by VU2DEV.

For another similar ckt after ARRL:- 2dev "The most reliable and simplest solution would be a 2N3866 multiple feedback amplifier as given in ARRL hand book! Gain is 17 dB and output is slightly more than +19 dBm as measured! Of course the resting current is high almost 63 Milli amps @13.8 volts! Multiple feed back: Emitter an unbypassed 5.6 ohms and collector to base 560 ohms feedback resistor! Base to ground is 270 ohms and emitter bypassed resistor is 18/22 ohms! Collector bifilar transformer to match 50 ohm low impedance output!   "

Tuned Amplifier with filter
 I have used this stage with my VWN transmitter with premixed oscillators, 14194 DSB exciter, Few TRF radio and more projects

(will update this with redrawn picture.
to be continued.....


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