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This version (07 Dec 2018 13:30) was approved by amiclaus.The Previously approved version (17 Aug 2017 18:04) is available.Diff

Activity 7. Zero gain amplifier (BJT)

Objective:

In the design of a circuit it is important to take into account the wide variation in certain device values from one to another. A central objective of the designer is to desensitize the circuit to these variations to produce a circuit which meets the specifications across all possible conditions. One aspect of design which is common to nearly all circuits is the establishment of stable bias or operating point levels. This seemingly minor portion of the design can provide the most challenging and interesting circuit problems. Many bias generators are centered around the generation of currents to operate the core of the circuit. Current generated from simple resistors and diodes or diode connected transistors connected across the power supply will vary approximately proportional to the variation of the supply voltage. This variation in the resulting bias current is frequently undesirable.

This is to introduce a current “mirror” in Activity 8 which has an output which had been desensitized to variation in input current. To understand this circuit, it is helpful to examine the behavior of a “zero-gain amplifier”.

Materials:

ADALM2000 Active Learning Module
Solder-less breadboard
1 - 2.2KΩ Resistor (or any similar value)
1 - 47Ω Resistor
1 - small signal NPN transistor (2N3904 or SSM2212)

Directions:

The breadboard connections are as shown in the diagram below. The waveform generator W1 output drives one end of resistor R1. Resistor R2 is connected between the base and collector of transistor Q1 with the other end of resistor R1 connected to the base as well. The emitter of Q1 is grounded.

Figure 1 Zero Gain Amplifier

Hardware Setup:

The waveform generator 1 should be configured for a 1 KHz triangle wave with 3 volt amplitude and 1.5V offset. Connect scope Channel 1 to display output W1 of the AWG generator. The Single ended input of scope channel 2 (2+) is used to measure alternately the base and collector voltage of Q1.

Figure 2 Zero Gain Amplifier Breadboard Circuit

Procedure:

Remembering back to the common emitter amplifier in the previous section, if RL is set equal to re then the gain A will be -1. If the base is connected to the top of resistor RL then the gain from the base to the collector (bottom of RL) will be -1. Also, neglecting the collector emitter output impedance of the transistor the gain from the top of load resistor RL to the collector (bottom of RL) will be +1. Thus the net gain superimposing both paths will be 1 - 1 = 0.

In the figure below we have a transistor biased into conduction with a collector voltage which is less than the base voltage by kT/q, (equal to Ic times 47Ω) and is essentially constant with input voltage changes applied from the AWG generator.

Figure 3 Scopy Plot VBE

Figure 4 Scopy Plot VCE

Figure 5 Excel Plot comparing VBE and VCE

Figure 6 Excel VBE and VCE vs. collector current

Questions:

What are the relative gains of the two paths when the collector current is less than and greater than the “ideal” zero gain value?

Improved VBE Multiplier, Applying the Zero Gain Concept

As we explored in Activity 3, there are often circuits which require that a voltage greater than 1 VBE be generated. Here we explore in more detail three additional ways to accomplish this.

VBE times 2 version 1:

The obviously simple thing to do would be to just use two diode connected transistors in series.

Materials:

1 - 1KΩ Resistor
2 - small signal NPN transistor (2N3904 or SSM2212)

Directions:

The breadboard connections are as shown in figure 7 below. The output of the AWG generator drives one end of resistor R1 as well as the 2+ input of scope channel 2. The emitter of Q1 is connected to ground. The base and collector of Q1 are connected to the emitter of Q2. The base and collector of Q2 are connected to the other end of R1 and to the 2- input of scope channel 2 and the 1+ input of scope channel 1.

Figure 7 VBE circuit

Hardware Setup:

Figure 8 VBE Breadboard circuit

The waveform generator should be configured for a 1 KHz triangle wave with 3 volt amplitude and 1.5V offset. Both scope channels can be set to 200mV per division.

Procedure:

Figure 9 Scopy Voltage vs. current

Figure 10 Voltage vs. current

You should also confirm that the voltage characteristic measured at the collector, base of transistor Q1 is the same as was measured in activity 3.

VBE times 2 version 2:

A second option would be to use two resistors as a voltage divider. This could produce an output that is the addition of fractions of a VBE to the VBE of Q1.

Materials:

1 - 1KΩ Resistor
2 - 10KΩ Resistors
1 - 5KΩ Variable resistor ( a 500Ω pot would be preferable if available )
1 - small signal NPN transistor (2N3904 or SSM2212)

Directions:

The breadboard connections are as shown in the diagram below. The output of the waveform generator drives one end of resistor R1 as well as the 2+ input of scope channel 2. The emitter of Q1 is connected to ground. Resistor R3 is connected between the base of Q1 and ground. One end of resistor R2 connected to the other end of R1 and to the 2- input of scope channel 2 and one end and the wiper of potentiometer R4 1+. The opposite end of R2 is connected to the base of Q1. The collector of Q2 is connected to the remaining end of R4 and the 1+ input of scope channel 1.

Figure 11 VBE Multiplier circuit

Hardware Setup:

Figure 12 VBE Multiplier Breadboard circuit

The waveform generator should be configured for a 1KHz triangle wave with 3 volt amplitude and 1.5V offset. Both scope channels can be set to 200mV per division.

Procedure:

Start out with variable resistor R4 set to its minimum value of nearly zero ohms. Observe the voltage vs. current characteristics of this configuration compared to version 1. There is a small extra current that flows in the two 10KΩ resistors before the transistor turns on. The voltage at 1mA is slightly higher and the slope of the curve is not as steep.

Figure 13 Scopy plot - R4 set to zero ohms

Figure 14 Excel plot - R4 set to zero ohms

Let’s apply the concept of the zero gain amplifier. Now adjust R4 and observe the slope of the curve change. At what value of R4 is the curve nearly vertical? Why is that value the correct value for “zero” gain?

Figure 15 Scopy plot - R4 set to approximately 100 ohms

Figure 16 Excel plot - R4 set to approximately 100 ohms

VBE times 2 version 3:

A minor variation on Version 2.

Materials:

1 - 1KΩ Resistor
1 - 10KΩ Resistor
1 - 5KΩ Variable resistor ( 500Ω pot would be preferable if available )
2 - small signal NPN transistor (2N3904 or SSM2212)

Directions:

The breadboard connections are as shown in the diagram below in figure 17. Version 3 is made from version 2 by removing 10KΩ resistor R2 and replacing it with diode connected NPN Q2 as shown.

Figure 17 Version 3 of VBE multiplier

Hardware Setup:

Figure 18 Version 3 of VBE multiplier Breadboard Circuit

The waveform generator should be configured for a 1KHz triangle wave with 3 volt amplitude and 1.5V offset. Both scope channels can be set to 200mV per division.

Procedure:

Again start out with variable resistor R4 set to its minimum value of nearly zero ohms. Observe the voltage vs. current characteristics of this configuration compared to version 2. There is a small extra current that flows in the one 10K resistors after Q1 turns on and until both Q1 and Q2 are on. The voltage at 1 mA is slightly lower and the slope of the curve is steeper more like version 1.

Figure 19

Figure 20

Again, let’s apply the concept of the zero gain amplifier. Now adjust R4 and observe the slope of the curve change. At what value of R4 is the curve nearly vertical? Why is that value the correct value for “zero” gain?

Figure 22

Figure 22

Optional extra credit problem:

Figure 23

How would you modify the values of R2 and R4 in version 2 ( figure 11 ) to produce a stabilized 1.0 volt output?

Answer: using a potentiometer for R2 the above curve was generated with R2 = 5.55KΩ and R4 = 69.8Ω.

Resources:

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university/courses/electronics/electronics-lab-7.txt · Last modified: 07 Dec 2018 13:29 by amiclaus