![]() Making the signal stronger often helps it cope with driving following stages. Don’t have an aneurysm trying to understand it all immediately.īottom line is that the cathode follower doesn’t embiggen the voltage from the grid, but it does add the current through the tube to make the signal stronger. The cathode follower can’t make the voltage changes at the cathode any bigger than the changes at the grid because it otherwise affects the difference between them and the difference is what determines electron flow. Ultimately, the output voltage (taken across the cathode resistor) follows the grid voltage up and down by about the same amount that the grid itself moves up and down because that’s all it can do. Changes in voltage on the grid (holding the cathode constant) have a reinforcing effect on this flow, but changes in voltage at the cathode (holding the grid constant) have an opposing effect. The difference in voltage between the grid and the cathode is what lets the tube control the flow of electrons. This epic battle between signal and bias is also at the core of the concept of feedback in tube amps. The two opposing forces are in a tug of war and, more or less, cancel each other out in the cathode follower. The opposite happens if the grid sees a negative signal: fewer electrons passed through the tube would lead to lower voltage across the cathode resistor (thus the grid becomes less negative in relation to it and wants to let through more current). The cathode voltage wants to increase, but if this happens, the grid’s voltage in relation to it would become more negative (which would let less current through the tube and force the voltage across the cathode resistor back down). Now if a positive change in the grid (making it less negative) allows another 5mA through the tube, what is the new voltage drop across the cathode resistor? What’s the voltage dropped across the 10,000 ohm cathode resistor when the circuit draws 15mA and has a 300V supply? It is the difference between the grid voltage and the cathode voltage that allows a tube to modulate the current through it.Ĭonsider the numbers in the example. The grid’s negative relation to the cathode is achieved by biasing the tube. When the grid is very negative, it doesn’t allow many electrons (current) through and when the grid is not quite so negative, it lets more electrons through. We know that the grid will have a more negative voltage potential than the cathode because otherwise it starts attracting the cathode’s electrons instead of allowing them to zip to the anode. We’re going to ignore the anode resistor for the moment. ![]() It’s at the connection to the cathode resistor instead of the connection to the anode resistor. Note how the output in this diagram is in a different place than what you’ve seen in amplification stages. ![]() It’s called the cathode follower and it doesn’t amplify sh!t, at least not in the voltage embiggening sense. Besides the grounded cathode amplifier, there’s another very common arrangement for a tube that you are sure to run across. ![]()
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