Tetrode Transistor - What is it?
Tetrode Transistor - Working Principe
We've all used n-p-n and p-n-p bipolar transistors with three connections but I had never heard of transistor tetrodes until I was researching this article. Tetrode transistor have four leads and come in different variations.
Remarkably they have two control elements instead of one, that's two base (b) connections for bipolar transistors and two gate (G) connections for MOSFETs, as indicated by the circuit symbols in Figure 1. Let's take a closer look at them and see what makes them special.
A tetrode in electronics is any device that has four active electrodes but usually the term refers to tetrode vacuum tubes which have two grids instead of one. The extra grid, called the screen grid, lowers the interplate capacitance when compared to a conventional triode tube which increases the tube's frequency range.
Transistor tetrode aren't an exact replacement for tetrode vacuum tubes but they also are made to reduce parasitic capacitance and increase their operating frequency range. A bipolar transistor tetrode is made by adding another base connection on the opposite side of the silicon like the left side of Figure 2. The right side image shows how a dual-gate MOSFET is constructed.
I've mentioned capacitance quite a bit but why does it matter? Well, designing amplifiers using discrete transistors is always a constant battle to get enough bandwidth, gain and input to output isolation. One of the reasons why this can be a challenge is the Miller Effect and how it can affect an amplifier.
The Miller Effect is when an amplifier's input to output capacitance is amplified by the amplifier gain which will then increase its equivalent input capacitance. In transistor amplifiers, it's usually the parasitic capacitance inherent to the transistor that must be overcome if you want to maximize the amplifier bandwidth.
One way to minimize the Miller Effect is to use two transistors to make a cascode amplifier like in Figure 3. One transistor is used as a transconductance amplifier which is then followed by a current buffer transistor. Having two amplifier stages helps to minimize the Miller Effect by isolating the amplifier input capacitance of the lower transistor from the buffer stage. And it turns out that transistor tetrodes are an excellent choice for small-signal cascode amplifiers that also need high frequency bandwidth.
The most common type tetrode transistor is the dual gate MOSFET which has some other interesting uses in RF circuits. Famous ones from the 1980s and ranking high in RF Design's Hall of Fame are the European BF96x/BF98x series, and from the US, the 40673 and 3N211. For example, a simple FET mixer will connect the local mixer output and the RF output to the gate of a FET using some extra components to isolate the mixer from the RF.
A dual gate MOSFET can eliminate those isolation components, with its gates already isolated from each other. Dual gate MOSFETs also work well in automatic gain control (AGC) circuits because you can bias one gate and feed the signal to the other. Then the bias voltage can be the control input and control the overall transistor gain.
I hadn't heard of tetrode transistor before today, but they are definitely a very interesting and weird component, especially with their uses in RF circuitry. Maybe you will find a use for them in your next project.
Elektor Electronics, 2015, January & February
