We spent the first our of this morning lesson in the laboratory. We have have to find the curve of amplify of our circuit.
We know from what we study in the theory that we would have bad value in high frequency (more than 1 Mhz) because of the parassite capacitance of the transistor. In the laboratory we found that kind of decrasing curve.
In the previous lecture we saw that we can improve the trend inserting an inductor L to compensate the effect of the parassite capacitance in high frequency, therefore we inserted that inductor and we could find better performance.
In the second part of the lecture Josè Maria explained to us the concept of positive regeneration. The basic idea is that we want to obtain an higher amplify. This idea is patended by Armstrong in 1915 and we have to understand that like in the principle of the electric age the engineers want to obtain the maximum with the minimum, in this case we want the maximum amplify with the minimum number of transistor (at the principle vacuum tube).
The way to obtain this positive regeneration is based on the idea that after we receive the signal we can emitting it with an antenna and then we re-received it.
There are two fundamentals problems: the first one is that we have to re-receive the signal in a constructively way (adding in phase) and the second problem is the saturation.
In practice to build up this "emitting antenna" we have only to add a little solenoid in the same ferrite kernel that we used for the antenna-tuner (paying attention to the phase), and a controlled resistence.
Controlling the resistence we can modify both the sensibily and the selectivity of our circuit (improving both at the same time) and we can avoid the problem of saturation.
martedì 26 ottobre 2010
giovedì 21 ottobre 2010
21/10/2010
Trying the transistor.
Today we start the lesson seeing that if we put our information voltage signal in series with the batteries we don' t obtain the gain that we desidered. Therefore in this montage our transistor "only" act a controlled switch.
Josè Maria said to us that we have to put our signal in parallel with the source power hence from base to ground and we have to add a capacitor that doesn't change the circuit in the DC mode.
Using the incremental model and the fasorial representation we arrived to calcute the amplify in this case: in principle we can choose the parameter to obtain the amplify that we want.
But there is a problem, the high we want the amplify the high result the impedence at the input of our amplifier, and remembering the previous lecture this is exactly what we don't want because we need to preserve the sensitivity of the antenna-tuner stage!
The problem of this high input resistance is well know in literature with the name of Miller effect. Fortunately there is a very clever solution to delete the undesidered Miller effect and hence we to replace the Rb (resistence in the base terminal) with a series of two Rb/2 resistence and in the middle a capacitor.
Jose' Maria showed us two other improvement for our circuit. The first is to insert a inductor L to compensate the effect of the parassite capacitance in high frequency, the second improvement is to insert in the emitter terminal a little resistence Re that doesn't afflict the polaritation circuit but that is seen multiplied for Beta in input (is fine for the sensitivity!).
We spent the last thirthy minutes of the lecture in the laboratory. We built the polaritation circuit, we turn on the power and we measured the real parameters of the circuit. We found a Collector voltage of Voq=4.58 V (theorically is 4.5 V) and a Base voltage of Vbq=0.61 V (teorically 0.6 V), with this mesured value we can calculate the various current and also the amplify factor that is A=169,99 that is in line with the theory.
Today we start the lesson seeing that if we put our information voltage signal in series with the batteries we don' t obtain the gain that we desidered. Therefore in this montage our transistor "only" act a controlled switch.
Josè Maria said to us that we have to put our signal in parallel with the source power hence from base to ground and we have to add a capacitor that doesn't change the circuit in the DC mode.
Using the incremental model and the fasorial representation we arrived to calcute the amplify in this case: in principle we can choose the parameter to obtain the amplify that we want.
But there is a problem, the high we want the amplify the high result the impedence at the input of our amplifier, and remembering the previous lecture this is exactly what we don't want because we need to preserve the sensitivity of the antenna-tuner stage!
The problem of this high input resistance is well know in literature with the name of Miller effect. Fortunately there is a very clever solution to delete the undesidered Miller effect and hence we to replace the Rb (resistence in the base terminal) with a series of two Rb/2 resistence and in the middle a capacitor.
Jose' Maria showed us two other improvement for our circuit. The first is to insert a inductor L to compensate the effect of the parassite capacitance in high frequency, the second improvement is to insert in the emitter terminal a little resistence Re that doesn't afflict the polaritation circuit but that is seen multiplied for Beta in input (is fine for the sensitivity!).
We spent the last thirthy minutes of the lecture in the laboratory. We built the polaritation circuit, we turn on the power and we measured the real parameters of the circuit. We found a Collector voltage of Voq=4.58 V (theorically is 4.5 V) and a Base voltage of Vbq=0.61 V (teorically 0.6 V), with this mesured value we can calculate the various current and also the amplify factor that is A=169,99 that is in line with the theory.
mercoledì 20 ottobre 2010
19/10/2010
Putting transistor in the active region.
After seeing another time how the transistor works we want to stress the fact that the nobel idea in the transistor is that little variations in the input imply big variations in the resistence in the output because there is a strong sensitivity in the voltage Base-Emitter.
Transistor comes from the words transfer + resistor that means that we can transfer the same current from an high resistence to a resistence that we can choose.
If we want to obtain the ransistor effect we have to put the transistor in the active region which means that we need a positive voltage both for the Collector-Base and for Base-Emitter.
If we want to check if the transistor in a circuit is in active region we have to check if at leat one of these current are positive: emitter, collector or base.
In the class we saw some proposals about how put a transistor in the active zone, in the simpliest cases we found some problem about the instability of the circuit due to temperature, the dependence from the value Beta of different transistor or the need for 2 batteries to aliment the circuit.
The last example that we have seen is made with only one batteries, with four resistance and it doesn't depend from the different value Beta of transistor (Note that also if the model of the transistor is the same we can find very different value of the parameter Beta).
In the last part of the lecture we developed an incremental model for study circuit with transistors and we start talking about the possibility of insert our voltage signal in parallel with the voltage Base-Emitter.
After seeing another time how the transistor works we want to stress the fact that the nobel idea in the transistor is that little variations in the input imply big variations in the resistence in the output because there is a strong sensitivity in the voltage Base-Emitter.
Transistor comes from the words transfer + resistor that means that we can transfer the same current from an high resistence to a resistence that we can choose.
If we want to obtain the ransistor effect we have to put the transistor in the active region which means that we need a positive voltage both for the Collector-Base and for Base-Emitter.
If we want to check if the transistor in a circuit is in active region we have to check if at leat one of these current are positive: emitter, collector or base.
In the class we saw some proposals about how put a transistor in the active zone, in the simpliest cases we found some problem about the instability of the circuit due to temperature, the dependence from the value Beta of different transistor or the need for 2 batteries to aliment the circuit.
The last example that we have seen is made with only one batteries, with four resistance and it doesn't depend from the different value Beta of transistor (Note that also if the model of the transistor is the same we can find very different value of the parameter Beta).
In the last part of the lecture we developed an incremental model for study circuit with transistors and we start talking about the possibility of insert our voltage signal in parallel with the voltage Base-Emitter.
lunedì 18 ottobre 2010
14/10/2010
The second stage.
The last lecture we saw how to make a transformer in our circuit to see a lower input impedance of the HF amplifier. Now we have to build this amplifier!
Beacuse we are in high frequency we can't use an operational amplifier and we have to remember and to exploit the propierties of the transistor.
Josè Maria explain us that to better understand the proprierties of the transistor we have to revise the feautures of the diode.
First of all we understand that a diode is made from a semiconductor (silicon), more precisely there are two different region doped P or N joint togheter.
After we have to observe that whenever we have a diode in a circuit, at that time the circuit is no longer linear, and so we saw the curve voltage vs corrent.
In practice we want to work with a linear circuit and therefore we saw how to approssimate a diode with a linear circuit.: we have to distinguish two different stage, one when the voltage is upper than the thereshold voltage and the second when the voltage is lower.
After that is necessary to see what happen when the voltage is moving a little bit from the fixed point used for the approximation. We saw that we have to image that there is a voltage generator and a resistence.
In summary if we have to study a circuit with diodes we have to proceed in two step: in the first step we find the operating point of the diode and in the second step we study what happen with little variation (we also exploit the proprierty of superposition).
After studied the diode we started to see how works a transistor. A transistor is made by 3 regions (normally N-P-N), we observed that the P-region is very very thin and so we can't approssimate the operation of the transistor like simply 2 diodes. The transitor has three terminals: Emitter, Collector and Base.
We saw that a little variations in the voltage of B-E correspond in an high variations in the current C-E, therefore if we insert a resistance we obtain an high variation in its terminal. Therefore we found the amplifiy effect of the diode!
The last lecture we saw how to make a transformer in our circuit to see a lower input impedance of the HF amplifier. Now we have to build this amplifier!
Beacuse we are in high frequency we can't use an operational amplifier and we have to remember and to exploit the propierties of the transistor.
Josè Maria explain us that to better understand the proprierties of the transistor we have to revise the feautures of the diode.
First of all we understand that a diode is made from a semiconductor (silicon), more precisely there are two different region doped P or N joint togheter.
After we have to observe that whenever we have a diode in a circuit, at that time the circuit is no longer linear, and so we saw the curve voltage vs corrent.
In practice we want to work with a linear circuit and therefore we saw how to approssimate a diode with a linear circuit.: we have to distinguish two different stage, one when the voltage is upper than the thereshold voltage and the second when the voltage is lower.
After that is necessary to see what happen when the voltage is moving a little bit from the fixed point used for the approximation. We saw that we have to image that there is a voltage generator and a resistence.
In summary if we have to study a circuit with diodes we have to proceed in two step: in the first step we find the operating point of the diode and in the second step we study what happen with little variation (we also exploit the proprierty of superposition).
After studied the diode we started to see how works a transistor. A transistor is made by 3 regions (normally N-P-N), we observed that the P-region is very very thin and so we can't approssimate the operation of the transistor like simply 2 diodes. The transitor has three terminals: Emitter, Collector and Base.
We saw that a little variations in the voltage of B-E correspond in an high variations in the current C-E, therefore if we insert a resistance we obtain an high variation in its terminal. Therefore we found the amplifiy effect of the diode!
venerdì 8 ottobre 2010
07/10/2010
Selectivity vs. sensibility.
We saw that we need more amplification to achieve the 0.3 V. So we have to insert an amplificator HF, but there is a problem, beacause of this amplifier the bandwidth of our antenna-tuner-band pass filter widens, and the amplitude of the peak decreased.
There is two important parameter that we have to consider, the sensibility of our system and the selectivity, there first is connected with the capability of detect poor power signal and the second is directly linked with the ability of the filter of cutting out the other station near the station that we want to hear. Is easier to hear that a radio system detect more than one station at the same time that see that the radio hasn' t a fine sensibility.
So we want first of all to preserve the bandwith of our filter, the most simple way to reduce the effect of the resistence of the amplificator HF is to implement a transformer so we can see an higher resistence. (Note that we lose in sensibility.)
Next we have to make this trasformer real in our circuit. To do it we roll (10 times) another wire above our primary coil, and we want to obtain in theory a parameter for the transormar equal to n=60/10=6. So we went to the laboratory and after made it we make some measure and we find that n=6.06. The resistence of the amplificator in HF will be seen multiplied by n.
martedì 5 ottobre 2010
05/10/2010
Measuring our coil.
Our aim of today is to measure the performance and some values of the coil that we made the last time.
The first things that we observeb is that we can measur the values of inductance L and the parasite resistance Rs from some value that we can see in the oscilloscope, we can calculate L from the value of the frequency of the peak and then with this value and the value of the max amplitude we can calculate Rs.
Like Heisenberg said in his uncertainty principle we can't measure some values without modifing that values, only beecause we are trying to mesure with an instrument. So our target is to minimize the effect of our instrument.
Jorgè Maria showed us a low capacitance probe that is able to "put" only 12 pF o capacity in our mesuring set-up. The price that we have to pay is that the amplitude is scaled by a 10 factor.
Now we were ready to go to the laboratory and test our coil and calculate L and Rs. We made this experiment for two position of the coil respect the kernel of ferrite so we can have an idea of the range of the values. Unfortunately me and my companion can't make the second mesure with the coil in a side of the ferrite because our coil doesn't slide well in the ferrite. The value that we obtain with the coil in the middle is L=310 uH and Rs=22 Ohm.
What happen? Why Rs is so hight? Tthe problem is that at high frequency (in our case around 1 Mhz) the skin effect is high and the copper becomes not a so good conductor.
With an Rs so high the performance of our antenna-filter-amplificator is worse. And so we have less amplification and a more large bandwidth.
Finally we observe that if we want to add an other amplificator in HF we have to minimize the effect of the resistence that is seen at the terminals otherwise we have, another time, a worsening in the performance.
Our aim of today is to measure the performance and some values of the coil that we made the last time.
The first things that we observeb is that we can measur the values of inductance L and the parasite resistance Rs from some value that we can see in the oscilloscope, we can calculate L from the value of the frequency of the peak and then with this value and the value of the max amplitude we can calculate Rs.
Like Heisenberg said in his uncertainty principle we can't measure some values without modifing that values, only beecause we are trying to mesure with an instrument. So our target is to minimize the effect of our instrument.
Jorgè Maria showed us a low capacitance probe that is able to "put" only 12 pF o capacity in our mesuring set-up. The price that we have to pay is that the amplitude is scaled by a 10 factor.
Now we were ready to go to the laboratory and test our coil and calculate L and Rs. We made this experiment for two position of the coil respect the kernel of ferrite so we can have an idea of the range of the values. Unfortunately me and my companion can't make the second mesure with the coil in a side of the ferrite because our coil doesn't slide well in the ferrite. The value that we obtain with the coil in the middle is L=310 uH and Rs=22 Ohm.
What happen? Why Rs is so hight? Tthe problem is that at high frequency (in our case around 1 Mhz) the skin effect is high and the copper becomes not a so good conductor.
With an Rs so high the performance of our antenna-filter-amplificator is worse. And so we have less amplification and a more large bandwidth.
Finally we observe that if we want to add an other amplificator in HF we have to minimize the effect of the resistence that is seen at the terminals otherwise we have, another time, a worsening in the performance.
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