Lesson 44 - The Triode as Applied to a Circuit
Knowing what a diode is, or how a triode works is of little use unless you have some practical knowledge of how it can be applied within a circuit. We are going to begin with a VERY basic schematic of an early transmitter. Do NOT try to build this at home! It probably won't work, and you may violate Federal Laws ( FCC regulations ) or injure yourself in the process. I will let you know when it is time to begin building projects. ( Yes, this course will come with a practicum ). I will also begin to explain the theory and operation of some new components.
When ever we are looking at a schematic diagram, we must look at it from 2 directions. Top to Bottom, and Left to Right:
First: No circuit can operate without some kind of Power, In this schematic, we have 2 batteries shown operating the circuit.
Second: The purpose of most circuits is to move some kind of signal. Signal flow normally goes from Left to Right.
Before you can fully understand the circuit, I will have to give you a brief introduction to microphone construction and theory.
In the following illustration, I show a breakdown of how 3 different types of microphones work. In all three types, when we speak into the microphone, the sound waves from your voice create vibrations in the air. When these sound waves reach the microphone, they vibrate the diaphram of the microphone, which is usually a very thin layer of flexible plastic. This diaphram works like the eardrum in your ear, and vibrates back and forth at the same frequency ( pitch ) and amplitude ( volume level ) of the sound waves that stimulate it. The diaphram in all three types move left and right as shown. The third pictures a 3 dimensional representation of a magnetic type microphone for ease of understanding.
In the case of the Condenser type, a condenser is basically a capacitor. Here we have 2 electrodes, separated by an air dialectric. When the diaphram moves, it moves one of the plates, which changes the distance between the plates. Since the distance between the plates governs the capacitance of a capacitor, by moving one plate, we change the capacitance of the capacitor. The changes in capacitance are directly proportional to the frequency and amplitude of the sound waves picked up by the diaphram. So the electrical signal coming out of the microphone reflects the sound waves picked up by the diaphram.
Carbon microphones work in similar fashion to Condensor microphones, except that instead of an air dialectric, we have carbon, which when compressed, changes its resistance instead of its capacitance. In short, we have a variable resistor, which changes reflect the incoming sound.
Finally, a dynamic, or magnetic microphone has a moving coil, which is moved by the diaphram when it vibrates. The hair like strands of wire in the coil move back and forth with the incoming sound, cutting the magnetic field of the permanent magnet. As you recall, when a wire cuts the field of a magnet, an electrical current is induced in the wire. When the coil moves, it generates a small electric current, which changes with the incoming sound.
Many microphones, such as the Carbon and Condenser type require power, whether via battery or "phantom power" to operate. Because a Dynamic microphone actually generates an AC current, it needs no outside power source.
Now let's get back to our diagram. Here we actually have 3 circuits. Let's begin with the first, and simplest, which I hope will allow you to get the idea of how to read a schematic diagram. On the far left of the diagram is this small section, which includes 3 components. There is a microphone on the left, a coil on the right and a battery at the bottom. The battery provides power for the microphone to operate.
The positive side of the battery is connected to the microphone directly. The negative side passes DC current through the coil to the other side of the microphone. Think of the microphone as a very thin membraned capacitor, with air acting as a dilectric. As we speak into the microphone, the vibration of the sound waves from our voice generates an alternating current, which varies directly in amplitude ( volume level )and frequency ( pitch ) with the changes of our voice. This alternating current is applied to the coil, causing it to set up an induced electromagnetic field, which rises and colapses, changing along with our speach.
Now on to the second part of the circuit... the Tube Amplifier! First let's go over power, then signal. The tube, being the active device in this circuit, is powered by the battery. The Positive side of the battery produces a positive voltage, which passes through the plate coil to the PLATE of the Tube. The Plate, having a positive potential, is ready to begin attracting electrons. We'll say we have a BIG battery, and it puts out 200 Volts.
The CATHODE of the tube is directly connected to the negative side of the battery, and begins to emit an electron cloud, which is attracted to and caught by the PLATE. We now have a flow of current from CATHODE to PLATE. Note that on the bottom right hand corner of our diagram we have another symbol ( 3 horizontal lines gradually getting smaller ). This is the symbol for the "ground" potential of the chassis, and is usually considered to be "0 Volts".
Now at this point, you might ask, "What is the voltage on the Control Grid?" This would be a very good question. Because of the combined resistances of the grid coil and grid resister, the grid must be at some voltage higher than that of the cathode. So if we say that the Plate is at 200 Volts, and the Cathode is at 0 volts, the grid must be somewhere in between. Another way of saying this is that the grid is positive with respect to the cathode. But because a coil has very little resistance at DC (battery), it wouldn't be TOO much higher than the 0 Volt Anode voltage. We'll estimate and say 1 or 2 volts.
Now we have 2 more terms to learn:
We now can say that the tube is working, but it is just sort of idling along, sort of like a car sitting at a traffic light. The motor is running, but it isn't going anywhere. When a tube has the proper voltages to operate, we say that it is "properly biased", and BIAS means voltage in most electronics work.
Also, whenever a tube is running, but in an idle state, with no signal applied, we say that it is in its QUIESCENT state. ( think "quiet" ).
Now let's apply a signal and see what happens. The signal from the microphone circuit is magnetically coupled to the amplifier circuit by way of 2 coils - the microphone coil, and the grid coil.
The mike coil is physically placed very close to the grid coil. When two coils are so close, that the magnetic field from the first coil is induced upon the second coil, we have what is called a TRANSFORMER.
Often 2 coils are actually wound on the same coil form. These coil forms can have an air dilectric, or may be ferrite ( iron ) based, which increases the amount of energy transferred from one coil to the other.
With a signal applied across the grid coil, it travels along the wire to the grid. The signal voltage alters the grid voltage at the rate of frequency and amplitude that is picked up by the microphone. This in turn increases and decreases the current which moves from the anode to the plate of the tube as described in the previous section. The output from the tube is an exact replica of the input of the microphone, except that it is greatly amplified ( made louder ), and is flipped to look like a mirror image.
So now that we have an amplified signal, what are the other components in the circuit for? Let's go over them one by one:
The Grid Biasing Resistor:
The grid resistor sets the bias voltage for the grid, so that it never reaches the two taboo states - cutoff and saturation. When a tube is in cutoff, it doesn't have current flow from anode to plate. When it is saturation, any change in signal at the grid won't be seen at the plate, because it will be at maximum current flow constantly. Both of these conditions can cause distortion of the signal, and are usually* unwanted. It is for this reason that we need to control the bias voltage to the grid by using a grid bias resistor.
The Plate Coil: Why didn't we simply connect the positive terminal of the battery directly to the plate? Answer: Because the output signal of the tube would then affect the battery current, which would affect the battery voltage, which would affect the tube operation. Bad Scene! Recall that a coil PASSES low frequencies ( D. C. is about as low as you can get ), but blocks the affects of AC. By using a plate coil, we can freely pass the DC to bias the plate, without the output signal affecting our power supply.
The Plate Coupling Capacitor: The purpose of this guy is simply to block the DC from the power supply from getting into the next circuit, while allowing the AC signal to pass on. Hence, it is called a "coupling capacitor", because it couples 2 circuits together.
After the coupling capacitor, we have the final output section of our transmitter. In the output section, there is a radio frequency signal generator, a transformer, and an antenna. Our amplified audio signal is coupled to the "final" section by way of the coupling capacitor. It then goes into the antenna output transformer. On the other side of the transformer is our radio frequency generator. The radio frequency generator creates a radio signal ( we won't go into how just yet ), which is coupled across the transformer to the audio side. The two signals mix in the transformer, and are sent out the antenna into the atmosphere. Now let's review:
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