Transistor amplifier   - Page 1
Forum section: electronics / projects

author: yalect
author_rank: Regular Contributor
timestamp: March 01, 2015, 09:49:39 am
content: Hello,I would like to ask you about my following circuit, some use it like RF part.What is the roll of the capacitor C4?C2 is for positive feedback how it’s calculated and if we increase the valueOf L1 connected to the emitter of the transistor, the feedback will increase?Thank you

author: MK
author_rank: Regular Contributor
timestamp: March 01, 2015, 12:27:40 pm
content: Looks like an oscillator to me

author: Zero999
author_rank: Super Contributor
timestamp: March 01, 2015, 01:37:07 pm
content: Perhaps this should have been posted in the beginners section.Yes it looks like an oscillator.Have you built the circuit?

author: yalect
author_rank: Regular Contributor
timestamp: March 01, 2015, 03:11:43 pm
content: Hello,Yes, it looks like oscillator and work as Colpitts oscillator but it’s a main part used for RF receiverThanks

author: Zero999
author_rank: Super Contributor
timestamp: March 01, 2015, 05:47:30 pm
content: Quote from: yalect on March 01, 2015, 03:11:43 pmDo you have the full schematic?I don't see how it's Colpitts which uses a tapped capacitor for feedback.It looks like a regenerative receiver. Q1 appears to be configured in common base with R5 and C6 forming a low pass filter with a cut-off of 23.4kHz which will block the RF on the output.Presumably R1 reduces the Q of the tank circuit so it doesn't oscillate continuously.

author: yalect
author_rank: Regular Contributor
timestamp: March 02, 2015, 07:34:55 am
content: Hello,Thank you for your replyThere are some circuit have the same work base of the Colpitts oscillator but not using a taped capacitor the feedback takes a way from the collector directlyThat circuit feeding second transistor throw the low pass filter (R5,C6) Q2 configured in common emitter, the filter takes it’s signal from the collector of the Q1 with LC circuit which is the output of the oscillator, C4 maybe for negative feedbackWhat about the value of C2 how it’s calculated and increasing the L1 will increase the value of positive feedback

author: Zero999
author_rank: Super Contributor
timestamp: March 02, 2015, 11:09:10 am
content: Are you talking about the typical kind of transmitter used in simple wireless microphones?It's not Colpitts. It looks like it is but it isn't because the feedback is not taken from the centre tap.Here's a link to a site which gives a more detailed description of the oscillator circuit and how to calculate the values.http://sm0vpo.altervista.org/begin/bug_calc.htmBear in mind that the circuit you posted is probably set up so it will not oscillate, unless there's an external signal.

author: Paul Price
author_rank: Super Contributor
timestamp: March 02, 2015, 10:07:14 pm
content: The poster's original circuit is a schematic of a super-regenerative FM receiver first published in Popular Electronics or Radio-Electronics magazine? around 1957-1958 with a then new hot RF transistor like a 2N502A or 2N384 that was able to work at these 100mHz+ high frequencies.C4 feeds back a portion of the RF carrier when tuned to a FM channel to the base of the transistor and so the FM is slope-demodulated by the germanium diode D1 and the demodulated audio is amplified by the transistor. The function of R1 is to both provide a audio frequency output load to the transistor and also voltage divider for the RF feedback to the base.R5 and C6 are a low-pass filter for the audio output, could easily drive a crystal earphone to a very hear able volume.I built one of these a few years ago and the sensitivity and sound quality is really fantastic for such a simple circuit.Whoever thought up this simple circuit was a genius! The values of the original circuit were optimized for best performance.Value of the RF choke in the emitter circuit is critical and had been optimized, and in the original circuit the choke was made by winding #32 wire around a resistor, so it was bypassed by a 100-ohm to 470 ohm resistor. Too little or too much inductance here and it just won't work.