Read Microsoft Word - Activity_10_emitter_follower.doc text version

Activity 10. Emitter follower

Objective: To investigate the simple NPN emitter follower amplifier also sometimes referred to as the common collector configuration. Materials: 1 ­ 2.2K Resistor ( RL ) 1 ­ small signal NPN transistor ( Q1 ) Directions: The breadboard connections are shown in the diagram below. The output of function generator AWG1 is connected to the base terminal. The collector terminal is connected to the positive ( +4V ) supply. The emitter terminal is connected to both the 2.2 K load resistor and Scope input A2+( SE ). The other end of the load resistor is connected to the negative ( -4V ) supply. To measure the input to output error, channel 2 of the scope can be used differentially by connecting A2+ to the base of Q1 and A2- to the emitter.

Emitter Follower Mobile Studio Setup: The function generator AWG1 should be configured for a 1KHz Sine wave with 4 volt peak to peak amplitude and 0 offset. The Single ended input of scope channel 2 (A2SE ) is used to measure the voltage at the emitter. The Scope should be set with channel 1 set to display AWG1 and channel 2 set to display A2SE. When measuring the input to output error, channel 2 of the scope should be set to display A2DIF.

3 2 1 0 -1 -2 -3 0 0.0005 0.001 0.0015 0.002 0.0025 Input Output

Procedure: The incremental Gain ( Vout / Vin ) of the emitter follower should ideally be 1 but will always be slightly less then 1. The gain is generally given by the following equation:

A=

RL RL + re

From the equation we can see that in order to obtain a gain close to one we can either increase RL or decrease re. We also know that re is a function of IE and that as IE increases re decreases. Also from the circuit we can see that IE is related to RL and that as RL increases IE decreases. These two effects work counter to each other in the simple resistive loaded emitter follower. Thus to optimize the gain of the follower we need to explore ways to either decrease re or increase RL without effecting the other. Looking at the follower in another way, because of the inherent DC shift due to the transistor's Vbe, the difference between input and output should be constant over the intended swing. However, because of the simple resistive load RL, the emitter current IE increases and decreases as the output swings up and down. We know that Vbe is a ( exponential ) function of IE and will change approximately 18 mV ( at room temperature ) for a factor of 2 change in IE. In this +2V to -2V swing example the minimum IE = 2V/ 2.2K or 0.91 mA to a maximum IE = 6V / 2.2K or 2.7mA. Which will result in a 28 mV change in Vbe. This observation leads us to the first possible improvement in the emitter follower. The current mirror from activity 5 is now substituted for the emitter load resistor to fix the amplifier transistor emitter current.. A current mirror will sink a more or less constant current over a wide range of voltages. This more or less constant current flowing in the transistor will result in a more or less constant Vbe. Viewed another way, the very high output resistance of

the current source has effectively increased RL while re remains at a low value set by the current. Materials: 1 ­ 3.3K Resistor 1 ­ small signal NPN transistor ( Q1 ) 2 ­ small signal NPN transistors ( Q2, Q3 ) selected for best Vbe matching

Improved Emitter Follower

Input - Output Error

6.0E-03 4.0E-03 2.0E-03 0.0E+00 Volts -2.0E-03 -4.0E-03 -6.0E-03 -8.0E-03 -1.0E-02 0 0.0005 0.001 0.0015 Time 0.002 0.0025 Resistor Error Current Source Error

Emitter follower output impedance

Objective: An important aspect of the emitter follower is to provide power or current gain. That is to say drive a lower resistance ( impedance ) load from a higher resistance ( impedance ) source. Thus it is instructive to measure the emitter follower output impedance. Materials: 1 ­ 4.7K Resistor 1 ­ 10K Resistor 1 ­ 500 ohm variable resistor 1 ­ small signal NPN transistor ( Q1 ) Directions: The circuit configuration below adds a resistor R2 to inject a test signal from AWG1 into the emitter ( output ) of Q1. The input, base of Q1, is grounded through a 500 ohm potentiometer. More on why the adjustable resistor later.

Mobile Studio Setup: The function generator AWG1 should be configured for a 1KHz Sine wave with 2 volt peak to peak amplitude with the offset set equal to minus the Vbe of Q1 ( approximately -0.65V ). This injects a +/- 0.1mA ( 1V/10K ) current into Q1's emitter. Scope input A2+ measures the change in voltage seen at the emitter. Procedure: The red line in the plot below shows the measured result with the adjustable resistor set to its minimum, i.e. 0 ohms. The voltage at the emitter actually goes down slightly as the test input goes up, and goes up slightly when the test input goes down. This would indicate that the output resistance of the emitter follower is negative in this case. Why is this happening?

Emitter Follower Output Impedance

0.75 0.25 -0.25 Input -0.75 -1.25 -1.75 0 0.0005 0.001 Time 0.0015 0.002 0.0025 Short to Gnd Resistor = re=36

The green line in the plot above shows the measured result with the adjustable resistor set to a value greater than 0 ohms. The student should adjust the resistor starting at 0 ohms and increase the value while observing the waveform. It should flatten out as the resistance in the base increases. The student should stop at the point where the line has just flattened out and measure the value of the potentiometer. What is the value? Why is that value significant? The nominal emitter current in Q1 is ( 4V ­ 0.65 ) / 4.7K or 720uA. We can calculate re from this current as 26mV/720uA or 36 ohms. How does this re compare to the value measured for the potentiometer? Low Offset Follower All the follower circuits we have investigated so far have a built in offset of ­Vbe. The circuit shown next uses the Vbe shift up of a PNP emitter follower to partially cancel the Vbe shift down of an NPN emitter follower.

Materials:

1 ­ 6.8K Resistor 1 ­ 10K Resistor 1 ­ 0.01uF Capacitor 1 ­ small signal PNP transistor ( Q1 ) 3 ­ small signal NPN transistors ( Q2, Q3, Q4 ) Directions: The breadboard connections are shown in the diagram below. The output of function generator AWG1 is connected to the base terminal of PNP transistor Q1. The collector terminal of Q1 is connected to . The emitter terminal is connected to both resistor R1 and the base terminal of

Volts

NPN transistor Q2. Scope input A2+( SE ) is connected to both the emitter of Q2 and the Collector of Q4. The emitters of both Q3 and Q4 are connected to the negative ( -4V ) supply.

Mobile Studio Setup: The function generator AWG1 should be configured for a 1KHz Sine wave with 2 volt peak to peak amplitude with the offset set equal to 0. Scope input A2+ is set to 500mV / Div . Procedure:

Low Offset Follower

2 1.5 1 0.5 Volts 0 -0.5 -1 -1.5 -2 0 0.0005 0.001 Time 0.0015 0.002 0.0025 Input Output

Error

0.0255 0.025 0.0245 Volts 0.024 0.0235 0.023 0.0225 0 0.0005 0.001 Time 0.0015 0.002 0.0025 Error

A problem suffered by the simple emitter follower can be seen when it drives a capacitive load. The rise time of the output can be relative fast as the emitter current is limited only by beta times the base current that can be supplied by the source driving the base. The fall time can be much slower and is limited by either the emitter resistor or current source.

Materials:

1 ­ 2K Resistor 1 ­ 2.2K Resistor 1 ­ 10K Resistor 1 ­ 0.01uF Capacitor 3 ­ small signal PNP transistor (Q2, Q3, Q4 ) 3 ­ small signal NPN transistors ( Q1, Q5, Q6 ) The circuit shown here uses feedback to adjust the current in the emitter follower as the current in the load changes. The current to pull the output negative can be as much as N times ( the gain of the NPN mirror ) the current in PNP Q3.

Output

0.6 0.4 0.2 0 -0.2 -0.4 -0.6 0 0.00005 0.0001 0.00015 0.0002 0.00025 Output

1 Volt Fall Time

0.6 0.4 0.2 Volts 0 -0.2 -0.4 -0.6 4.8E-05

Series1 Series2

5.0E-05

5.2E-05

5.4E-05 5.6E-05 Time

5.8E-05

6.0E-05

6.2E-05

6.4E-05

4 Volt Fall Time

2.5 2 1.5 1 0.5 Volts 0 -0.5 -1 -1.5 -2 -2.5 4.8E-05 Series1 Series2

5.3E-05

5.8E-05

6.3E-05 Time

6.8E-05

7.3E-05

7.8E-05

An alternate approach to improving the emitter follower is to reduce the effective re through negative feedback. Reducing re can be addressed by adding a second transistor to increase the

negative feedback factor by increasing the open-loop-gain. The single transistor is replaced by a pair with 100% voltage feedback to the emitter of the first transistor. This is often referred to as a complementary feedback pair. The value of R2 is crucial to good linearity, as it sets the IC of transistor Q1, and also determines its collector loading

Materials:

1 ­ 2.2K Resistor 1 ­ 10K Resistor 1 ­ small signal NPN transistor ( Q1 ) 1 ­ small signal PNP transistor ( Q2 )

Complementary Feedback Pair Emitter Follower A minor addition to the complementary feedback pair emitter follower can provide a gain greater than 1. Resistor R3 is added between the collector of PNP Q2 and the emitter of NPN Q1. The output is now taken at the collector of Q2. The gain is approximated by the ratio of R3 to R1, Gain = (R1+R3)/R1. In this example it is about 3.2. Materials: 1 ­ 1K Resistor 1 ­ 4.7K Resistor 1 ­ 2,2K Resistor 1 ­ small signal NPN transistor ( Q1 ) 1 ­ small signal PNP transistor ( Q2 )

Follower with gain greater than 1

1.5 1 0.5 0 -0.5 -1 -1.5 -2 -2.5 0 0.0005 0.001 0.0015 0.002 0.0025 Input Output

In addition to the gain ( or as a result of it ) the DC level of the output is shifted positive as compared to the gain of 1 version. The next plot normalizes out the DC offset.

0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 -1.2 0 0.0005 0.001 0.0015 0.002 0.0025 Input AC Output AC

To confirm that the gain is indeed about 3.2 the next plot divides the output by the gain and compares that to the input. In this example the actual gain is 3.16, most likely due to the resistor values not being exact.

0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 0 0.0005 0.001 0.0015 0.002 0.0025 Input AC Output/Gain

Questions: What limits the amount of gain greater than one that this circuit can produce? What could be added to the circuit to remove / restore the DC levels at the input and output of this circuit?

Information

Microsoft Word - Activity_10_emitter_follower.doc

12 pages

Report File (DMCA)

Our content is added by our users. We aim to remove reported files within 1 working day. Please use this link to notify us:

Report this file as copyright or inappropriate

981580