LT1228CS8 datasheet(16/20 Pages) LINER

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LT1228

Single Supply Wien Bridge Oscillator

LT1228 • TA14

–

CFA

680

Ω

V+

10k

Ω

10k

Ω

1.8k

160

Ω

1000pF

160

Ω

µF

V+

470

Ω +

µF

100

Ω

0.1

µF

Ω

2N3906

6V TO 30V

f = 1MHz

FOR 5V OPERATION SHORT OUT 100

Ω RESISTOR

VO = 6dBm (450mVRMS)

2nd HARMONIC = –38dBc

3rd HARMONIC = –54 dBc

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 RG to set

the DC gain of the current feedback amplifier to unity.

The transconductance amplifier is used as a variable resis-

tor 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 160

Ω

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 600

µA 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 • TA15

–

CFA

330

Ω

V+

30pF

Ω

VO = 10dB

AT VS = ±5V ALL HARMONICS 40dB DOWN

AT VS = ±12V ALL HARMONICS 50dB DOWN

9.1k

750

Ω

V–

4.3k

4.7

µH

10k

0.1

µF

V–

2N3904

2N3906

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 transcon-

ductance 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.

APPLICATI

TYPICAL

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