6.Experiments

Constant-current diode

Anode

Cathode

A normal PN-junction diode is included in series with the JFET to protect the transistor against damage from reverse-bias voltage, but otherwise the current-regulating facility of this device is entirely provided by the ¯eld-e®ect transistor.

COMPUTER SIMULATION

Schematic with SPICE node numbers:

Netlist (make a text ¯le containing the following text, verbatim):

JFET current regulator vsource 1 0

rload 1 2 4.5k j1 2 0 3 mod1 rlimit 3 0 1k

.model mod1 njf

.dc vsource 6 12 0.1

.plot dc i(vsource)

.end

SPICE does not allow for "sweeping" resistance values, so to demonstrate the current regulation of this circuit over a wide range of conditions, I've elected to sweep the source voltage from 6 to 12 volts in 0.1 volt steps. If you wish, you can set rload to di®erent resistance values and verify that the circuit current remains constant. With an rlimit value of 1 k-, the regulated current will be 291.8 ¹A. This current ¯gure will most likely not be the same as your actual circuit current, due to di®erences in JFET parameters.

Many manufacturers give SPICE model parameters for their transistors, which may be typed in the .model line of the netlist for a more accurate circuit simulation.

5.16Di®erential ampli¯er

PARTS AND MATERIALS

²Two 6-volt batteries

²Two NPN transistors { models 2N2222 or 2N3403 recommended (Radio Shack catalog # 276-1617 is a package of ¯fteen NPN transistors ideal for this and other experiments)

²Two 10 k- potentiometers, single-turn, linear taper (Radio Shack catalog # 271-1715)

²Two 22 k- resistors

²Two 10 k- resistors

²One 100 k- resistor

²One 1.5 k- resistor

Resistor values are not especially critical in this experiment, but have been chosen to provide high voltage gain for a "comparator-like" di®erential ampli¯er behavior.

CROSS-REFERENCES

Lessons In Electric Circuits, Volume 3, chapter 4: "Bipolar Junction Transistors" Lessons In Electric Circuits, Volume 3, chapter 8: "Operational Ampli¯ers"

LEARNING OBJECTIVES

²Basic design of a di®erential ampli¯er circuit.

²Working de¯nitions of di®erential and common-mode voltages

SCHEMATIC DIAGRAM

ILLUSTRATION

INSTRUCTIONS

This circuit forms the heart of most operational ampli¯er circuits: the di®erential pair. In the form shown here, it is a rather crude di®erential ampli¯er, quite nonlinear and unsymmetrical with regard to output voltage versus input voltage(s). With a high voltage gain created by a large collector/emitter resistor ratio (100 k-/1.5 k-), though, it acts primarily as a comparator: the output voltage rapidly changing value as the two input voltage signals approach equality.

Measure the output voltage (voltage at the collector of Q2 with respect to ground) as the input voltages are varied. Note how the two potentiometers have di®erent e®ects on the output voltage: one input tends to drive the output voltage in the same direction (noninverting), while the other tends to drive the output voltage in the opposite direction (inverting). This is the essential nature of a di®erential ampli¯er: two complementary inputs, with contrary e®ects on the output signal. Ideally, the output voltage of such an ampli¯er is strictly a function of the di®erence between the two input signals. This circuit falls considerably short of the ideal, as even a cursory test will reveal.

An ideal di®erential ampli¯er ignores all common-mode voltage, which is whatever level of voltage common to both inputs. For example, if the inverting input is at 3 volts and the noninverting input at 2.5 volts, the di®erential voltage will be 0.5 volts (3 - 2.5) but the common-mode voltage will be 2.5 volts, since that is the lowest input signal level. Ideally, this condition should produce the same output signal voltage as if the inputs were set at 3.5 and 3 volts, respectively (0.5 volts di®erential, with a 3 volt common-mode voltage). However, this circuit does not give the same result for the two di®erent input signal scenarios. In other words, its output voltage depends on both the di®erential voltage and the common-mode voltage.

As imperfect as this di®erential ampli¯er is, its behavior could be worse. Note how the input signal potentiometers have been limited by 22 k- resistors to an adjustable range of approximately 0 to 4 volts, given a power supply voltage of 12 volts. If you'd like to see how this circuit behaves without any input signal limiting, just bypass the 22 k- resistors with jumper wires, allowing full 0 to 12 volt adjustment range from each potentiometer.

Do not worry about building up excessive heat while adjusting potentiometers in this circuit! Unlike the current mirror circuit, this circuit is protected from thermal runaway by the emitter resistor (1.5 k-), which doesn't allow enough transistor current to cause any problem.

5.17Simple op-amp

PARTS AND MATERIALS

²Two 6-volt batteries

²Four NPN transistors { models 2N2222 or 2N3403 recommended (Radio Shack catalog # 276-1617 is a package of ¯fteen NPN transistors ideal for this and other experiments)

²Two PNP transistors { models 2N2907 or 2N3906 recommended (Radio Shack catalog # 2761604 is a package of ¯fteen PNP transistors ideal for this and other experiments)

²Two 10 k- potentiometers, single-turn, linear taper (Radio Shack catalog # 271-1715)

²One 270 k- resistor

²Three 100 k- resistors

²One 10 k- resistor

CROSS-REFERENCES

Lessons In Electric Circuits, Volume 3, chapter 4: "Bipolar Junction Transistors" Lessons In Electric Circuits, Volume 3, chapter 8: "Operational Ampli¯ers"

LEARNING OBJECTIVES

²Design of a di®erential ampli¯er circuit using current mirrors.

²E®ects of negative feedback on a high-gain di®erential ampli¯er.

SCHEMATIC DIAGRAM

As you test this noninverting ampli¯er circuit, you may notice slight discrepancies between the output and input voltages. According to the feedback resistor values, the voltage gain should be exactly 2. However, you may notice deviations in the order of several hundredths of a volt between what the output voltage is and what it should be. These deviations are due to imperfections of the di®erential ampli¯er circuit, and may be greatly diminished if we add more ampli¯cation stages to increase the di®erential voltage gain. However, one way we can maximize the precision of the existing circuit is to change the resistance of Rprg . This resistor sets the lower current mirror's control point, and in so doing in°uences many performance parameters of the op-amp. Try substituting di®erence resistance values, ranging from 10 k- to 1 M-. Do not use a resistance less than 10 k-, or else the current mirror transistors may begin to overheat and thermally "run away."

Some operational ampli¯ers available in prepackaged units provide a way for the user to similarly "program" the di®erential pair's current mirror, and are called programmable op-amps. Most opamps are not programmable, and have their internal current mirror control points ¯xed by an internal resistance, trimmed to precise value at the factory.

5.18Audio oscillator

PARTS AND MATERIALS

²Two 6-volt batteries

²Three NPN transistors { models 2N2222 or 2N3403 recommended (Radio Shack catalog # 276-1617 is a package of ¯fteen NPN transistors ideal for this and other experiments)

²Two 0.1 ¹F capacitors (Radio Shack catalog # 272-135 or equivalent)

²One 1 M- resistor

²Two 100 k- resistors

²One 1 k- resistor

²Assortment of resistor pairs, less than 100 k- (ex: two 10 k-, two 5 k-, two 1 k-)

²One light-emitting diode (Radio Shack catalog # 276-026 or equivalent)

²Audio detector with headphones

CROSS-REFERENCES

Lessons In Electric Circuits, Volume 3, chapter 4: "Bipolar Junction Transistors" Lessons In Electric Circuits, Volume 4, chapter 10: "Multivibrators"

LEARNING OBJECTIVES

² How to build an astable multivibrator circuit using discrete transistors

SCHEMATIC DIAGRAM

ILLUSTRATION