LT1228MJ8TRPBF 查看數據表（PDF） - Linear Technology

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LT1228MJ8TRPBF

100MHz Current Feedback Amplifier with DC Gain Control

Linear Technology 

LT1228

TYPICAL APPLICATIONS

Single Supply Wien Bridge Oscillator

100Ω

V+

10kΩ

10kΩ

+

10µF

2N3906

6V TO 30V

V+

7

3+

gm

2–

4

470Ω +

10µF

5

1+

CFA

8–

RF

680Ω

0.1µF

6

51Ω

VO

1.8k

160Ω

RG

20Ω

+

10µF

1000pF

1000pF 160Ω

50Ω

f = 1MHz

VO = 6dBm (450mVRMS)

2nd HARMONIC = –38dBc

3rd HARMONIC = –54dBc

FOR 5V OPERATION SHORT OUT 100Ω RESISTOR

LT1228 • TA14

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

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

9.1k

1k

V+

3+

7

gm

2–

5

4

V–

4.7µH

30pF

4.3k

1+

CFA

8–

1k

330Ω

6 51Ω

VO

750Ω

50Ω

2N3906

2N3904

10k

0.1µF

V–

VO = 10dB

AT VS = ±5V ALL HARMONICS 40dB DOWN

AT VS = ±12V ALL HARMONICS 50dB DOWN

LT1228 • TA15

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.

1228fd

16

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