Datasheet

LT1228
16
1228fd
Single Supply Wien Bridge Oscillator
TYPICAL APPLICATIONS
LT1228 • TA15
–
+
–
+
g
m
CFA
V
O
8
3
2
1k
330Ω
4
1
6
V
+
7
1k
30pF
51Ω
50Ω
V
O
= 10dB
AT V
S
= ±5V ALL HARMONICS 40dB DOWN
AT V
S
= ±12V ALL HARMONICS 50dB DOWN
9.1k
750Ω
V
–
5
4.3k
4.7µH
10k
0.1µF
V
–
2N3904
2N3906
In this application the LT1228 is biased for operation from
a single supply. An artificial signal ground at half supply
voltage is generated with two 10k resistors and bypassed
with a capacitor. A capacitor is used in series with R
G
to
set the DC gain of the current feedback amplifier to unity.
The transconductance amplifier is used as a variable
resistor to control gain. A variable
resistor is formed by
driving the inverting input and connecting the output back
to it. The equivalent resistor value is the inverse of the
gm. This works with the 1.8k resistor to make a variable
attenuator. The 1MHz oscillation frequency is set by the
Wien bridge network made up of two 1000pF capacitors
and two 160Ω resistors.
For clean sine wave oscillation, the circuit needs a net gain
of one around the loop. The current feedback amplifier has
a gain of 34 to keep the voltage at the transconductance
amplifier input low. The Wien bridge has an attenuation of
3 at resonance; therefore the attenuation of the 1.8k resis-
tor and the transconductance amplifier must be about 11,
resulting in a set current of about 600µA at oscillation. At
start-up there is no set current and therefore
no attenuation
for a net gain of about 11 around the loop. As the output
oscillation builds up it turns on the PNP transistor which
generates the set current to regulate the output voltage.
12MHz Negative Resistance LC Oscillator
LT1228 • TA14
–
+
–
+
g
m
CFA
V
O
8
3
2
5
R
F
680Ω
R
G
20Ω
4
1
6
V
+
7
10kΩ
10kΩ
1.8k
160Ω
1000pF 1000pF
160Ω
+
10µF
+
10µF
V
+
470Ω
+
10µF
100Ω
0.1µF
51Ω
50Ω
2N3906
6V TO 30V
f = 1MHz
FOR 5V OPERATION SHORT OUT 100Ω RESISTOR
V
O
= 6dBm (450mV
RMS
)
2nd HARMONIC = –38dBc
3rd HARMONIC = –54dBc
This oscillator uses the transconductance amplifier as a
negative resistor to cause oscillation. A negative resistor
results when the positive input of the transconductance
amplifier is driven and the output is returned to it. In
this example a voltage divider is used to lower the signal
level at the positive input for less distortion. The negative
resistor will not DC bias correctly unless the output of the
transconductance amplifier drives a very low resistance.
Here it sees an inductor to ground so the gain at DC is
zero. The oscillator needs negative resistance to start
and that is provided by the 4.3k resistor to Pin 5. As the
output level rises it turns on the PNP transistor and in turn
the NPN which steals current from the transconductance
amplifier bias input.