Path: utzoo!utgpu!water!watmath!clyde!cbosgd!mandrill!neoucom!wtm
From: wtm@neoucom.UUCP (Bill Mayhew)
Newsgroups: sci.electronics
Subject: Re: R-C oscillator
Summary: try a twin T oscillator
Message-ID: <971@neoucom.UUCP>
Date: 24 Jan 88 17:51:07 GMT
References: <2507@ihlpe.ATT.COM>
Organization: Northeastern Ohio Universities College of Medicine
Lines: 45


My ancient copy of the ARRL handbook has simple little circuit for
a twin T single transistor circuit.  It is quite stable.  It
requires one transistor, 4 resistors and 4 capacitors.  One of the
caps is used for coupling the output.  Basically you use a low-pass
T circuit and a hig-pass T wired in parallel to feed back.  The
circuit oscillates at the freqency where there is maximal feedback.
At that frequency, the phase shifts of both T sections is 180
degress, permitting oscillation.

You can approximate the output frequency by graphically overlaying
the responses of the T sections.  Making the output fequency
adjustable is a hassle.

I'll give this a shot:

                        o (Vcc)
                        |
                        R-bais
                        |
          +-------------+--------+
          |                      |
          |         o (OUT)      | c
          |         |         b  |
          +----R1---+---R1--+----Q1
          |         |       |    |
          |         C1      |    | e
          |         |       |    gnd
          |         gnd     |
          |                 |
          +----C2---+---C2--+
                    |
                    R2
                    |
                    gnd

The output is taken at the R1-C1 junction.  Harmonic distortion in
minimized at that point.  To assure oscillation R2 ~= 0.1 * R1, and
C1 ~= 2 * C2.  Rbias should be ~2700 - 4700 ohms.

I found this on pg. 522 of my 1974 ARRL handbook.  I remember
building one with a 2N414 years ago.  Definitely a circuit for Old
Farts.  I never really did like the light bulb in a Wien bridge.

--Bill