Solid State Circuits 5 - LF Amplifiers & Buffers

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1. Single Emitter Follower

2. Dual Emitter Follower

3. Bootstrapped Emitter Follower

4. Source Follower

5. Source Follower and Bipolar Amplifier

6. Buffered LF Amplifier

Circuit 1 has a lower output current capability than circuit 2.



These figures apply to circuits 1 & 2

Input Level pk-pk

Output Distortion

1v

<0.1%

2v

<0.1%

3v

0.10%

4v

0.13%



Voltage gain

Just under 1

Frequency response

Flat to at least 1MHz

Input impedance

170K



Note:  Emitter and Source Followers can oscillate with inductive input loads like screened cables. If this occurs place a 100R resistor in series with the input close to the device.

This circuit uses a bootstrap feedback to increase the input impedance with a minor increase in output distortion at higher input levels.



Input Level pk-pk

Output Distortion

1v

<0.1%

4v

0.19%



Voltage gain

Just under 1

Frequency response

Flat to at least 1MHz

Input impedance

5M6

This circuit uses a Junction FET to provide a very high input impedance determined by the gate resistor.



Input Level pk-pk

Output Distortion

1v

0.52%

2v

1.15%

3v

2.25%



Voltage gain

Just under 1

Frequency response

Flat to at least 1MHz

Input impedance

1M0



The distortion should improve with a higher supply voltage subject to the device specification.

This circuit uses a Junction FET followed by a pnp amplifier and negative feedback to reduce the low level distortion in circuit 4.



Input Level pk-pk

Output Distortion

1v

0.12%

2v

0.25%

3v

2.07%



Voltage gain

1

Frequency response

Flat to at least 1MHz

Input impedance

1M0



Increasing the supply to 24V reduced the distortion at 2v ouptut to 0,12%.

-3dB frequency response:

17Hz - 1MHz


Gain

x90



Output level pk-pk

Distortion

2v

3.2% with 12v supply

4v

1.4% with 24v supply



Input impedance

170K

7. Single Stage LF Amplifier

Emitter Potentiometer Setting

Voltage Gain

Input Impedance (ohms)

Minimum

x5

7K

Halfway

x8.8

6K9

Maximum

x258

4K7

The minimum setting is with the slider at the 0v end.

As the collector has an almost constant current characteristic the output impedance is effectively the value of the collector resistor i.e. 4K7ohms.

Simple single stage amplifier measured at three settings of the emitter potentiometer. Capacitor values are for voice communications applications so will need to be increased for HiFi uses. Onset of clipping at output level = 7v pk-pk. Output clipping is not symmetrical so for minimum distortion the circuit should be used at output levels well below the onset of clipping. The input impedance is the impedance seen at the base in parallel with the base bias resistors. The BC109 is a low noise but older European design transistor which may be replaced with a more modern type with an equivalent specification.