# Analog Devices Wiki

This version (23 Jan 2020 21:23) was approved by Doug Mercer.The Previously approved version (23 Jan 2020 21:08) is available. This is an old revision of the document!

# Activity: The BJT connected as a diode

## Objective:

The purpose of this activity is to investigate the forward and reverse current vs. voltage characteristics of a bipolar junction transistor (BJT) connected as a diode.

## Materials:

1 - 1KΩ Resistor (or any similar value)
1 - small signal NPN transistor (2N3904)

## Directions:

The current vs. voltage characteristics of the base-emitter junction of an NPN transistor can be measured using the ADALM2000 Lab hardware and the following connections. Set up the breadboard with waveform generator, W1, attached to one end of resistor R1. Also connect scope input 2+ here. Connect the base and collector of Q1 to the opposite end of R1 as shown in the diagram. The emitter of Q1 is connected to ground. Connect scope input 2- and scope input 1+ to the base - collector node of Q1. (Scope input 1- can be optionally grounded as well).

Figure 1 NPN diode connection diagram

## Hardware Setup:

The waveform generator should be configured for a 100 Hz triangle wave with 6 volt amplitude peak-to-peak and 0 offset. The differential scope channel 2 (2+, 2-) measures the current in the resistor (and in the transistor). Scope channel 1 (1+) is connected to measure the voltage across the diode connected transistor. The current flowing through the transistor, is the voltage difference 2+ and 2- (which is the channel 2 voltage) divided by the resistor value (1KΩ).

Figure 2 NPN diode breadboard circuit

## Procedure:

Load the captured data in to a spreadsheet program like Excel and calculate the current. Plot the current vs. the voltage across the transistor (VBE). No current flows in the reverse direction. In the forward conduction region, the voltage, current relationship is logarithmic. If the current is plotted on a log scale the line should be straight.

Figure 3 NPN diode XY plot

Figure 4 NPN diode waveform

## Questions:

Derive the mathematical expression for the current, IC, given the voltage across the transistor VBE?

# Reverse Breakdown Characteristics

## Objective:

The purpose of this activity is to investigate the reverse break down voltage characteristics of the emitter base junction of a bipolar junction transistor (BJT) connected as a diode.

## Materials:

1 - 100Ω Resistor
1 - small signal PNP transistor (2N3906)

## Directions:

Set up the breadboard with the waveform generator output attached to one end of the series connected resistor 100Ω R1 and base and collector of Q1 as shown in figure 2. The emitter is connected to the negative 5 Volt fixed power supply. Scope channel 1 (1+) is connected to the base - collector node while 1- is connected to the emitter node. Scope channel 2 measures the voltage across R1 and thus the current through Q1. The PNP 2N3906 is chosen over the NPN 2N3904 because the PNP emitter base breakdown voltage is less than the +10 V max that can be generated using ADALM2000 while the NPN's is likely to be higher than 10 V.

Figure 5 PNP Emitter Base Reverse breakdown configuration

## Hardware Setup:

The waveform generator should be configured for a 100 Hz triangle wave with 10 volt amplitude peak-to-peak and 0 volt offset. Scope channel 1 (1+) is used to measure the voltage across the transistor. The setup should be configured with channel 2 connected across resistor R1 (2+, 2-). Both channels should be set to 1 V per division. The current flowing through the transistor is the voltage difference between 2+ and 2- divided by the resistor value (100Ω).

Figure 6 PNP Emitter breadboard circuit

## Procedure:

The Lab hardware power supplies limits the maximum voltage available to less than 10 volts. The emitter base reverse breakdown voltage of many transistors is larger than this. In the configuration shown voltages between 0 volts and 10 volts ( W1 peak to peak swing ) can be measured.

Figure 7 PNP Emitter waveform

Capture the scope waveforms and export them into a spreadsheet program such as Excel. For the 2N3906 PNP used in the example, the breakdown voltage of the emitter base junction is around 8.5 volts.

## Questions:

Disconnect the collector of Q1and leave it open. How does this change the breakdown voltage? Now connect the collector to the emitter. How does this change the breakdown voltage?

Try measuring the NPN 2N3904 emitter base reverse breakdown voltage. You can also check the emitter base breakdown voltage for the two power transistors, TIP31 and TIP32, which are included in the ADALP2000 Analog Parts Kit. Are they higher or lower than the PNP 2N3906 and is it lower than the +10 volts you can measure with this setup? If it is higher what could you add to the setup to allow you to measure higher breakdown voltages?

# Lowering the effective forward voltage of the diode

## Objective:

The purpose of this activity is to investigate a circuit configuration with smaller forward voltage characteristics than that of a bipolar junction transistor (BJT) connected as a diode.

## Materials:

1 - 1KΩ Resistor
1 - 150KΩ Resistor ( or 100KΩ in series with 47KΩ )
1 - small signal NPN transistor (2N3904)
1 - small signal PNP transistor (2N3906)

## Directions:

Set up the breadboard with waveform generator W1 attached to one end of the series connected resistor R1 and collector of NPN Q1 and the base of PNP Q2 as shown in the diagram. The emitter of Q1 is connected to ground. The collector of Q2is connected to Vn (-5V). The first end of Resistor R2 is connected to Vp (+5V). The second end of R2 is connected to the base of Q1 and the emitter of Q2. Single ended input of scope channel 2 (2+) is connected to the collector of Q1.

Figure 8 Configuration to lower effective forward voltage drop of diode

## Hardware Setup:

The waveform generator should be configured for a 100 Hz triangle wave with 8 volt amplitude peak-to-peak and 2 volt offset. Scope channel 2 (2+) is used to measure the voltage across the transistor. The current flowing through the transistor, is the voltage difference between scope input 1+ and 1- divided by the resistor value (1K?).

Figure 9 Lower effective forward voltage drop of diode - breadboard circuit

## Procedure:

The turn on voltage of the “diode” is now about 100mV compared to 650mV for the simple diode connection in the first example. Plot the VCE of Q1 as W1 is swept.

Figure 10 Lower effective forward voltage drop of diode - waveform

## Questions:

Could the collector of the PNP Q2 be connected to some other node such as ground? And what would be the effect?

The value of R2 sets the current in Q2. What is the effect of increasing or decreasing the value of R2?

# VBE Multiplier Circuit

## Objective:

Now that we have seen a way to make VBE effectively smaller, the purpose of this activity is to make VBE larger. Larger forward voltage characteristics than that of a single bipolar junction transistor (BJT) connected as a diode.

## Materials:

2 - 2.2KΩ Resistors
1 - 1KΩ Resistor
1 - 5KΩ Variable resistor, potentiometer
1 - small signal NPN transistor (2N3904)

## Directions:

Set up the breadboard with waveform generator W1 attached to one end of resistor R1 as indicated in figure 4. The emitter of Q1 is connected to ground. Resistors R2, R3 and R4 form a voltage divider with the wiper of potentiometer R3 connected to the base of Q1. The collector of Q1 is connected to the second end of R1 and the top of the voltage divider at R2. Scope channel 2 (2+) is connected to the collector of Q1.

Figure 11 VBE Multiplier configuration

## Hardware Setup:

The waveform generator should be configured for a 100 Hz triangle wave with 4 volt amplitude peak-to-peak and 2V offset. The Single ended input of scope channel 2+ is used to measure the voltage across the transistor. The setup should be configured with channel 1+ connected to display the output of generator W1 and channel 2+ connected to display the collector voltage of Q1. The current flowing through the transistor, is the voltage difference between the W1 measured by scope input 1+ and scope input 2+ divided by the resistor value (1KΩ).

Figure 12 VBE Multiplier breadboard circuit

## Procedure:

Starting with the potentiometer R3 set at the middle of its range the voltage at the collector of Q2 should be about 2 times VBE. With R3 set to its minimum the voltage at the collector should be 9/2 (or 4.5) times VBE. With R3 set to its maximum the voltage at the collector should be 9/7 times VBE.

Figure 13 VBE Multiplier breadboard waveform

Resources:

## Questions:

How does the voltage vs current characteristics of this VBE multiplier compare to those of a simple diode connected transistor?

Aside from the position of the pot wiper, do the values of R2, R3 and R4 effect the shape of the I vs V curve? To arrive at an answer try using values much larger and much smaller than those listed above. 