Why cathode follower

In other words, if the cathode is made more positive, the grid will effectively become more negative relative to it and, thus, less current will flow through it, which decreases the output voltage; on the other hand, if the cathode is made less positive, the grid will effectively become more positive relative to it and, thus, more current will flow, which increases the output voltage.

The greater the transconductance, the lower the output impedance, as the greater the transconductance, the greater the change in current due to a deviation in output voltage relative to input voltage.

Unfortunately, in spite of low distortion, a cathode follower can sound bad if sloppily designed or improperly used. The old textbook approach was to use a cathode resistor five times smaller in value than the load being driven; for example, for a k external load, use a 20k cathode resistor.

Another trap is assuming that cathode followers are great for driving capacitive loads; they aren't, as they work much better at driving purely resistive loads. Back in the s, tests were performed that showed if a grounded-cathode amplifier were configured with plate resistor whose value match the Zo of the equivalent cathode follower, then it outperformed the cathode follower at driving a capacitor at high frequencies.

Whenever I expect encounter a highly-capacitive load, I place a series resistor at the cathode follower's output, thus preventing the cathode from seeing the naked capacitance; and I always ensure that the idle current is robust enough to ensure adequate slew-rate. And never forget that a cathode follower can never symmetrically swing more current into the external load than its idle current draw, as it is strictly confined to single-ended operation; for example, if it idles at 1mA, then it can only swing 0.

After a while, these simple rules and guidelines become habit and designing a good cathode follower becomes second nature. In other words, if you are careful, you can easily get excellent performance from a cathode follower. This doesn't mean that the stock cathode follower cannot be modified or improved. For example, we can replace the cathode resistor with a constant-current source, either a tube-based or solid-state effort—or even an inductor. We can create a push-pull version by placing two tubes in series and using either the White cathode follower or the Broskie cathode follower topology.

Or we could retain the single-ended operation and use the Aikido cathode follower topology, which will null power-supply noise at its output. No, I didn't think so.

First go through all the voltages to reassure yourself that it could bias correctly. Second, note that both triode and PNP transistor will operate in anti-current phase to each other.

In other words, a positive input voltage will increase the triodes current conduction, but reduce the transistor's. The least obvious aspect is the two resistors that span between the cathode and the collector.

Why two resistors and not just one? In a push-pull circuit, we want to ensure equal current swings from both output devices. But the beefiest triode is no match for the wimpiest transistor in terms of transconductance.

So, the ohm resistor not only helps bias the triode, it serves to undo much of the transistor's gm, so that matches the triode's with the ohm cathode resistor in place. How well does this push-pull follower work? The test was 1Vpk a 1kHz into ohms. Unlike tubes, transistors can work with just few volts across them. For example, say we planned on driving ohm headphones instead. Such caps are expensive, frail, and clunky. In contrast, with the above circuit and a ohm load, we could use a super-high-quality Panasonic non-polarizied electrolytic coupling capacitor, such as the ECEA1CNU.

Even if we choose to further improve the signal transfer by adding a film bypass capacitor, a V film capacitor will cost much less and be much smaller than the same capacitor in a V form. I would also try something along these lines:. High-quality electrolytic capacitors are used back to back, with the 1M resistor biasing the capacitors up or is down?

Furthermore, low-voltage transistors are readily available. In other words, class-AB operation. Where the plain-Jane by the way, all the women named "Jane" I have met have been very nice looking indeed cathode follower is limited, is to its idle current. In b , the load resistor has to have a smaller value than that required for developing the bias. These latter arrangements are to be preferred to ensure that the output is not less than the input level, which is likely in the circuit at the top of the page.

The input impedance of a cathode follower is very high though in practice it may be limited by the presence of the grid leak across the input , while the output impedance is very low. Like this: Like Loading Follow Following.

Sign me up. Already have a WordPress. Log in now. They certainly pack a bigger punch than the added two and four watts, respectively, would account for. And this response is very musical, with just enough natural compression to sound and feel good. Again, these amps come across as bigger, beefier and bolder. Cathode follower? The Deluxe amps mix the best of traditional tube tones with state-of-the-art technical features, making your life as a guitar player more fun, easier — and better sounding — than ever before!

Take a bow, designers! Classic tube tone, cooked up with the finest ingredients and the tastiest traditional recipes, all ready to go right out of the box: 21 st century amps like these truly are a crossroads where tradition and modern-day innovation converge; where the cathode follower and FRFR coexist in the happiest of unions.

Are you bothered about whether your amp has a cathode follower? Does the technology behind guitar tone matter to you? Just click on this beautiful picture below to read our little TubeMeister 18 to TubeMeister Deluxe 20 case study:. They also feature cathode followers but not DC-coupled ones, if I remember correctly. Glad you enjoyed the post Jimi! I tend to agree in theory without knowing all of the circuit details that a Low Z higher current lower voltage before the signal is amplified would potentially have more thickness to it and could potentially give a thicker beefier sound to be amplified by the power section.

I think of current like a river. The stronger it is, the more things it can carry, including Tone. My problem, I just bought a TubeMeister 18 head and love it. It sure seems to get a lot of attention online. Can the 20 be that much better? This allows me to control the DI out to the PA system separately from my amps speaker volume which is great for monitoring myself. Is there a mod to make both outputs work at the same time?