TELE-TECH & - AmericanRadioHistory.Com

1 -
P a e /rom an ¿njineer teb00%
No. 36-Wien
By J. F. SODARO
Bridge Design
Fig. 1: Conventional Wien bridge circuit
THE Wien Bridge network shown
in Fig. 1 is used as a frequency
selective circuit in oscillators and
for audio frequency measurements.
When used with an oscillator,
this circuit is connected
to a suitable amplifier with regen-
erative feedback.
For the mea-
surement of frequency, the unknown
frequency is applied across
upper and lower terminals and a
null detector across right and left -
side terminals. In the design
of this network it is customary to
make RI= R_= R and C1 =C._=
C and R;5 /Ra = 2. For these conditions
the bridge is balanced at
the frequency given by
f =
2,RC
(I)
As an oscillator selective network,
R4 is an incandescent lamp
which controls amplitude by increasing
resistance with current
and thus approaching the bridge
balance value. In this case the
bridge is never quite balanced since
an exact balance would cut -off regenerative
coupling and stop oscillations.
The 'nomograph shown in Fig. 2
may be used to evaluate Eq. (1) .
To use this nomograph locate the
required frequency on an f scale
and pivot a straight -edge about this
point to select combinations of
values on the R and C scales that
give the desired frequency. If the
resistance value is fixed, place the
straight -edge on this R scale value
and on the variable condenser maximum
value on C. Read the low
frequency limit where the straightedge
crosses f. Now pivot the
straight -edge on R to minimum condenser
value on C. Read the high
frequency limit where the straightedge
crosses f. The R, C, and f
scales have split stems. Use left -
side scales to obtain magnitude and
approximate answers. Use right-
ZOOM
10M-
1 M-
10K -
R
10
=9
-= 8
100 -
10 -
7
FZ__ -
t
0.001 -
o 0.01 =
z O U -
W 0.1 =
N m -
w
I
0_
=
= J 10 =
=- 5 r
- U
_ 100 =
=4
- 3
-2
L1000 =
IO Kc
100 Kc =
I
MC =
IOMc=
f
side scales to obtain additional significant
figures.
As an example assume that R is
60,000 ohms and C is 0.005 µf. Find
the balance frequency if the ratio
of R.4 to RI is two -to -one. Using
left -side scale stems connect 60 k
on R to 0.005 µ on C with a straight
line. Estimate 500 cps where this
line crosses f. Next use right -side
scale stems and connect 6 on R and
5 on C with a line. Read 5.3 where
the line crosses f. Thus, the balance
frequency is 530 cps.
- 1.5
- 2
3
4
5
6
- 7
8
- 9
ç-1- - -
C- 2
= 3
4
6
7
r
I-
C
IAJ --10
.I,U -
U -
-Q----
D-
U
.OI,IJ -
0.001 AI 1000 ALU -
I00AJAJ -
10 ALU -
=8
- 7
= 6
5
= 4
-3
-2
J. F. SODARO, Registered Engineer, 2924
Selby Ave., Los Angeles 64, Calif.
TeleTech & ELECTRONIC INDUSTRIES June 1956
- I
1.5
IAJAJ -
- I
93