1. Purpose of the dissertation
2. Theoretical introduction
2.1 Parameters of operational amplifiers
2.2 Internal structure of the ULY7741N amplifier
2.3 Working systems of amplifiers
3. Technical documentation of the device
3.1 Principles of the power supply system
3.2 Principles of the control system
3.3 Description of the main board plug
3.4 List of used electronic elements
4. Measurement of the operational amplifiers parameters
4.1 Scheme 1. Amplification, Input differential resistance, Output resistance, Width of the frequency band
4.2 Scheme 2. Output summation resistance
4.3 Scheme 3. Common-mode rejection ratio
4.4 Scheme 4. Polarization input current
4.5 Scheme 5. Kit for the individual construction of amplifiers working systems
5. Conclusions
6. Bibliography
7. Presentation - the movie
1. Purpose of the dissertation
The device for examining the operational amplifier is designed for students to use during Electronic Laboratory. It is made for students to get to know some simple working systems, which are useful to measure basic parameters of the operational amplifier, and to get practical skills of creating of basic operational amplifier working systems.
2. Theoretical introduction
The first operational/differential amplifiers constructed in the 40's and 50's of the XX century were able only to perform basic mathematical operations, like addition, subtraction, integration, differentiation of signals - from this their name comes.
During the development process, the practical usage was expanding. A sudden increase of interest in operational gains appeared in connection with their mass production as monolithic integrated circuits having very good properties and not a high price. The operational amplifier by dint of its universal usage is now the basic, and most prevalent analog integrated circuit.
A definition - operational amplifier/gain, means an amplifier with direct feedbacks, with very high gain, and as a rule designed to work with external circuit of negative feedback, but properties of such circuit decide mainly about properties of the whole circuit. Most of operational amplifiers have symmetrical (differential) inputs and asymmetrical outputs (Figure 1.)
A. Main symbol, B. Basic substitute diagram
An input signal, that is carried between inputs of amplifier, is called the Uwer differential signal. The Uo output voltage is proportional to the value of a differential signal.
where:
U1, U2 - input voltages
Uo - output voltage
Kur - voltage gain of the amplifier with open feedback
Uwer - differential input voltage
I1, I2 - input currents
2.1. Parameters of the operational amplifier
a) Voltage gain with open feedback (Kur)
Voltage gain with open-loop feedback is the ratio of output voltage change (Uo) to input voltage change (U1-U2) causing it, so
Kur = dUo/d(U1-U2) [V/V], or Kur = 20log(dUo/d(U1-U2)) [dB].
b) Input disequilibrium voltage (Uwen)
It is a value of a permanent voltage that must be carried between differential inputs in order to cause a permanent output voltage to be zero.
c) Output disequilibrium voltage (Uwyn)
It is a value of a permanent voltage between output and the ground, when input connectors are linked with the ground.
d) Thermal factor of the input disequilibrium voltage
It is the ratio of an input disequilibrium voltage change to a temperature change causing it, which is expressed in
µV/oC
e) Input polarization current (Iwe)
It is an arithmetic mean of currents being carried to reversing and non-reversing inputs of the balanced amplifier, Iwe = (I1 + I2)/2.
f) Input disequilibrium current (Iwen)
It is the result of subtraction of direct currents that are carried to inputs when an output voltage is zero, that is
Iwen = I1 - I2, przy Uo = 0.
g) voltage gain factor of the common-mode signal (Kus)
of common-mode signal
It is the ratio of an output voltage change (dUo) to a voltage change of common-mode signal (dUs) causing it, so Kus = dUo/dUs.
h) Common-mode rejection ratio CMRR
An ideal operational amplifier should amplify a differential signal and completely dump a common-mode signal. The CMRR factor describes how much a real amplifier differs from an ideal one. It is defined as a ratio of Kur differential signal gain to common-mode signal gain,
that is CMRR = Kur/Kus [dB].
i) Differential input resistance (Rwer)
It is a resistance between reversing and non-reversing inputs, that is Rwer = |(U1 - U2)/(I1 - I2)|.
j) Summation input resistance (Rwes)
It is a resistance between linked inputs of amplifiers and the ground, that is Rwes = (U1 + U2)/(I1 + I2).
k) Output resistance (Rwy)
It is the ratio of an output voltage (Uo) to an output current (Io), when U1 = U2 = 0.
l) Width of transmitted frequency band (B)
It is a width of a frequency band measured from direct current (f=0) to frequency fg, when the value of gain decreases by 3dB in relation to the value of Kur for direct current.
Becuse of usage, it is important that Kur, CMRR, Rwe, B have the highest value, and the other parameters are the smallest.
2.2 Internal structure of the ULY7741N amplifier
The ULY7741N operational amplifier is a Polish equivalent to the µA741 operational amplifier created by the Fairchaid Semiconductor company.
The µA741 belongs to the second generation of amplifiers and till this moment it has been the most commonly used operational amplifier.
T1 - T7 transistors constitute an input stage of the amplifier. Current gains of T1 and T2 transistors are about 200. The bases of T3 and T4 transistors, and collectors of T1 and T2 (through T8 and T9 transistors) are fed from the T10 and T11 current source. Such a power supply system causes a current which fed the input stage to be independent from the current gain of T3 and T4 side transistors.
The second gain stage is the Darlington pair (T16 and T17). The transistor T17 is loaded by the current source (T12 and T13), which ensures high voltage gain of this stage.
Between the input and output of the second stage, there is the integrated compensatory capacitor (C1) of MOS type. By dint of a high input resistance of the second gain stage (about 1M[Om]), small capacity (about 30pF) is enough to get a proper inclination of the gain characteristic in the function of frequency.
At an output of the amplifier there is the conventional complementary system (T14 and T20) which works in the AB class by dint of polarizing it by a current with a value about 60µA.
This system is protected from exceedance of a value of output current in both directions.
The positive output current is limited to a value of 25mA.
When this value is exceeded, a voltage on the R9 resistor increases, which causes the T15 transistor to be unblocked.
When a value of negative current is too high, a voltage on the R11 resistor causes the T22 transistor to conduct.
In the ULY7741N amplifier we are allowed to compensate an input disequilibrium voltage by connecting a potentiometer between emiters of T5 and T6 transistors, end-pieces of which are put outside of a casing. Controlling by potentiometer (10k) causes unsymmetrization of T5 and T6 emiter currents.
2.3 Working systems of the amplifier
In this working system a negative parallel voltage feedback was applied (R2 resistor).
We assume that voltage gain of an amplifier with the open-loop feedback (Kur) goes to the infinity, hence an input differential voltage (Ur=Uo/Kur) goes to zero.
Because of a high input resistance of the amplifier the input current is very small (close to zero).
So, the potential in the Z point is approximately equal to the potential at non-reversing input, hence it is close to the potential of the ground, because no current goes through the R3 resistor.
Thus the Z point in called an apparent ground point.
Because an input current is close to zero, we assume that input currents are the same, and if we take into account that the potential in the Z point is equal to zero, basing on the Kirchhoff's law we obtain U1/R1 = -Uo/R2.
Thus we get the formula for the voltage gain of the reversing amplifier
Kuf = Uo/U1 = -I2R2/I1R1, hence Kuf = -R2/R1.
The input resistance of the reversing amplifier is equal to R1 (Rwe=R1), because the resistance which we can see between the apparent ground point and the ground is negligibly small. A value of R3 is taken to be equal to the resistance of R1 and R2 parallel connection. That results in having the best compensation of an error caused by a disequilibrium voltage.
In this working system (Fig.9) an input signal is carried to the non-reversing input, and a part of an output signal through a resistors divisor to the reversing input. So in this case a negative series voltage feedback was applied.
Similar to the previous example, we assume that Kur goes to the infinity, so Ur is close to zero.
From an assumption that input polarizing currents are equal to zero, it ensues that I1 and I2 currents are equal (I1=I2). Hence U2=I1R1, Uo=I1(R1+R2),
and thus the amplification of non-reversing amplifier equals to
Kuf = Uo/U2 = I1(R1+R2)/I1R1, Kuf = 1 + R2/R1.
The R3 resistor is applied in order to decrease the influence of polarizing currents. The input resistance of this system is very high.
The voltage follower (Fig. 10) is an amplifier with the gain value of 1. We receive this system from a non-reversing amplifier, by disconnecting the R1 resistor, and so applying a full negative feedback.
The system is characterised by a very high input resistance and small output resistance. A value of the R2 resistance must be chosen to be equal to an internal resistance of an input signal source.
We receive the differential amplifier from the connection of both reversing and non-reversing amplifiers.
Assuming ideal properties of an operational amplifier and marking voltages at reversing and non-reversing inputs as U1' i U2', we can write
a formula of currents balance (U2-U2')/R3 = U2'/R4 oraz (U1-U1')/R1 = (U1'-Uo)/R2.
When Kur goes to the infinity, the voltage U1' equals to U2' (U1'=U2'), and then an output voltage is equal to
Uo = (R1+R2)/(R3+R4)(R4/R1)U2 - (R2/R1)U1.
Chosing resistors so that R2/R1 = R4/R3, we obtain an output voltage which is proportional to a difference of input voltages
Uo = R2/R1(U2-U1).
Input resistances of both inputs are the same. A resistance of the first input equals to R1, and of the second R3+R4.
Similar to the reversing amplifier, the "Z point" is an apparent ground, hence the values of input currents are I1 = U1/R1, I2 = U2/R2, ... In = Un/Rn.
The I current that runs through the R5 feedback resistor is I = I1 + I2 +...+ In. The output voltage Uo = -RsI, hence Uo = -Rs(U1/R1 + U2/R2 +...+ Un/Rn).
If we take the same values of resistors R1 = R2 =...= Rn = R, we receive Uo = -Rs/R(U1 + U2 +...+ Un), and so we obtain algebraical voltage summation. The input resistances of the system being observed from each input are R1, R2,..., Rn respectively.
3. Technical documentation of the device
The constructed device was maximally minimised, which is confirmed by its tiny dimensions. A casing was made as a console with a separate main board and control panel. At the main board there were placed schemes of each measurement circuit, radio sockets functioning as measurement points and also an input generator socket of the BNC type.
The construction of the main board is an innovative solution, because of using white colour for the presentation of schemes on the black panel, which makes it more clear and consolidated with the casing, which is also black.
The control panel was placed at a right angle in relation to the main board, which makes work with the device easier and pleasant. On this panel there are buttons and 4 rotary potentiometers, the functioning of which is described in the schemes on the main board and further in this technical documentation.
Another innovative solution applied in the device is the control panel, which was made using digital technique.
Instead of standard switches small aesthetic unstable buttons were used, which because of their functions can be divided into 3 groups.
First is the STAND BY/ON button, the pressing of which switches the device on or turns it to the standby mode (this function will be talked-about later).
To the second group there belong buttons labelled 1 - 4 functioning as a switch of signal from generator to each measurement circuit.
To the third group there belong buttons labelled A - D functioning as switches, which close the routes in each measurement circuit.
Working modes of each element of the control panel are signalled with flaring of LEDs coloured the same as buttons.
The above mentioned standby function works in this way that after each putting of a main plug into a 230V socket the device turns into the standby mode.
It is a state where a main voltage is drawn only to signal this mode by red flaring of D11 LED, which is placed above the STAND BY/ON button.
Pressing of this button switches the device on, that is switches on voltages feeding all circuits, both of operational amplifiers and the whole control panel.
Next pressing of the button turns the device into the standby mode again.
The main advantage of the described function is that we do not have to use big 230V switches, which require great force comparing to an unstable button.
In this case the function of 230V switch is fulfilled by a relay controlled by a digital circuit and by the mentioned button.
Another interesting solution is the equipping of the main board with a wide plug placed inside the device. When access to electronic elements inside the device is necessary, during the disassembly of the main board the wide plug is disconnected and the main board is separate, which allows to have a free access to all elements inside the device.
In a rear part of the device there is a 230V socket, and thanks to its application the device is plugged into the 230V main system by means of a separate standard cable.
Anti-electric shock protection is assured by a fuse, which is placed in the rear part of the device, and by a material from which the casing is made. It is a textolith with very good dielectric qualities.
3.1 Principles of the power supply system operation
It is best to describe the principle on the following example (scheme of the power supply system in fig. 16):
We put a plug into a 230V socket. An alternating voltage is given through the fuse directly to a primary side of the TR1 transformer.
An alternating voltage on a secondary side of the transformer is straightened on its way through Graetz bridge and is given to the integrated circuit ULY7805 (US6) - stabilizer, which keeps a stable value of +5V at the output.
At the input and at the output of the stabilizer there are C12 and C13 condensers. The C12 condenser works as a filter, which smooths a pulsating voltage coming from the Graetz bridge. The C13 condenser compensates for sudden voltage decreases at the output of the stabilizer, which are caused by fast changes of load current. Next the +5V voltage is given to the US9 and US10 integrated circuits and to the integrating circuit, which is placed at the B input of mono-stable trigger UCY74121 (US9). The time constant of the R33 C3 integrating circuit delays a delivery of a rising slope to the B input in relation to a delivery of the supply voltage to US9 and US10 integrated circuits, so that the whole system would work properly. Next the US9 mono-stable trigger generates at the non-Q output short impulse (low level L), which lasts as long as the time constant of the R34 C4 circuit. This impulse is given to the non-R reset input of the D trigger (US10), which sets a low level (L) at the Q output. Then the high level (H) signal at the non-Q output of the D trigger polarizes one of the two anodes of the D11 diode responsible for emission of red light.
At this moment the system is in the standby mode and waits for pressing the STAND BY/ON button. The pressing of the button causes a rising slope to occur at the C input of the D trigger. The D trigger, which works in the T trigger configuration, changes a state at the Q output into an opposite one (at this moment high level H), which controls the T1 transistor and the transistor raises current flow through a coil of the Pg main relay and causes its joints to close. This state is signalled by emitting a green light by D11 LED. The R35 resistor assures a switch of the voltage at the C output of the D trigger from L to H (low to high level), each time the STAND BY/ON button is pressed. The C5 condenser dulls disturbances which appear during switching.
The SPg1 closed contact of the main relay causes the supply voltage to be delivered to the (US11) mono-stable trigger and to the whole circuits working in the control system. At the input of the US11 mono-stable trigger (similar to US9) there is the integrating circuit (R38 C6), which delays a delivery of a rising slope to the B input of the trigger in relation to a moment of delivery of the supply voltage of the D11 trigger.
The mono-stable trigger generates at the Q and non-Q outputs a short impulse, which lasts as long as the time constant of the R39 C7 circuit. The impulse is
used in the control system, in order to reset the triggers, that is to set L at their outputs. In order to do that both Q and non-Q outputs are used.
SPg2 - the second contact of the main relay causes the 230V voltage to be delivered to the primary side of the TR2 transformer. On the secondary side there are two windings, the common contact of which is connected to the ground, and the two other contacts are carried to the voltage stabilizer, which was built using ULY7815 and ULY7915 (US7, US8) integrated circuits with output voltages of +15V and -15V. These voltages are used to supply operational amplifiers.
3.2 Principles of the control system
The control system consists of 8 triggers of the D type, 4 of which (US14, US15) work in a dependant configuration and switch the signal from the generator input, and the other 4 function as closing contacts (US12, US13).
The D inputs of triggers were connected to non-Q outputs, which means that the D triggers work in a T trigger configuration, of which it is characteristic that each impulse given at the C clock input causes a change of state at the Q output into an opposite one.
The R8 - R15 resistors connected to the ground ensure the low state at the C input when the button contacts are open. Pressing of a button changes a state at the C input to high and causes the D trigger to work - the high state occurs at the Q output. This state controls the transistor and causes a current to flow through the coil of the relay, and in this way the active contacts to close.
Each high state at the Q output is signalled by flaring of the D12 - D19 LEDs. The R16 - R23 resistors limit a LEDs current.
To non-R (RESET) inputs of the D triggers a signal from the US11 mono-stable generator is given. At the moment of switching on, this circuit resets all the D triggers by a short impulse, which means that it sets the low level at their outputs.
In the circuit of generator signal switch (US14 and US15) there exists a combinative circuit which fulfils a function of dependant switching of P1 - P4 relays contacts.
This system works in a way that e.g. pressing button No. 3 resets triggers No. 1,2 and 4. Then the signal from a generator occurs only at the G3 output (Fig. 17). Next pressing e.g. button No. 1 resets triggers No. 2, 3 and 4 and switches the signal from a generator only to the G1 output. Next pressing the same button again opens the SP1 contact, and in this way the signal from the input of a generator is not given to any of G1 - G4 outputs.
The function which executes the algorithm above is shown in the figure below.
and logical functions of each reset inputs.
Because of the impossibility to buy in the market integrated circuits with 4-input AND gates or 4-inputs NOR gates it was necessary to use a greater number of gates. It is visible in the following 3 schemes, which are logically equivalent.
A. AND and NOT gates, B. NOR gate, C. NAND and NOT gates.
Finally, 4-input NAND gates (US16 and US17) and inverters (US18 and US19) were used.
3.3 Description of the main board plug
