Amplifiers and FETs
George continues his article series looking at all aspects of the basic structures that make semiconductors work. In Part 4, he shows you some useful discrete transistor circuits and then zeros in on the field effect transistors: Junction FETs and MOSFETs. The discussion includes an examination of some fundamental structures like cascode amplifiers and Darlington transistors.
Last month we exhausted the topic of transistor theory, so now let’s look at some useful transistor circuit topology. Figure 1a shows you a cascode (not cascade!) amplifier and two versions of Darlington transistor in Figure 1b and Figure 1c, respectively. The three configurations can be realized with either NPN or PNP or both transistors. The latter would be called a complementary pair.
The cascode amplifier (Figure 1a) can be found in many radio and higher frequency circuits. Compared with the common base amplifier—which is also used as high frequency amplifier—the cascode amplifier features higher input impedance, especially when a small resistor is placed between the Q1 emitter and ground. It also has a higher output impedance—both advantageous for interfacing with tuned circuits. And, last but not least, it has greater bandwidth because the cascode topology eliminates the Miller effect.
The Miller effect causes an increase of the equivalent input capacitance of inverting voltage amplifiers by amplifying the actual capacitance, in our case, between the base and the collector terminals. Predictably, the result is the reduction of the maximum frequency response. The Miller effect, by the way, affects all inverting voltage amplifiers, whether vacuum tube or solid state. It is expressed as:
where AV is the voltage gain of the amplifier and C the capacitance between the input and the output—in other words, base to collector in our example.
Because of the cascode amplifier’s ability to eliminate the Miller effect, you can often find it as the front end of RF receivers. Transistor Q1 works in common emitter configuration, feeding Q2 working as a common base amplifier. This amplifier can and often is realized with junction (JFET) and metal oxide (MOS) FETs. We’ll address that later when we discuss those semiconductors. The drawback of cascode amplifiers is that it’s comprised of two transistors with additional parts and usually a higher supply voltage is needed. But then the end result is a top-notch high frequency amplifier. To obtain maximum gain, the load RL should be as high as possible. In critical applications, a constant current source is used instead of the resistor RL.
THE DARLINGTON TRANSISTOR
The Darlington transistor (Figure 1b) is named after its inventor Sidney Darlington. It is simply a cascade of two transistors Q3 and Q4, NPN or PNP, in a single package. Its main advantage is a very high current gain, which is approximately the product of the two transistors’ gains—this is usually in the thousands. The price you pay for this feature lies in a doubling of the base-to-emitter voltage VBE and increased collector saturation voltage VCEQ4 by VBE, which leads to a higher power dissipation than a single transistor. The Darlington current gain is:
Unfortunately, the switching speed of the Darlington pair is reduced because Q3 cannot effectively inhibit the base current of Q4. This is to some degree alleviated by the base to emitter resistors integral within the transistor. A complementary pair—one NPN and the other PNP—can be used instead of two transistors of the same type as illustrated by Figure 1c. Either transistor can be NPN or PNP. The Darlington pair’s larger phase shift when compared with a single transistor may lead to a system instability.
Another important topology is the balanced, differential, emitter coupled pair—sometimes called a “long tailed pair.” It is shown in Figure 2. You’ll find it in many applications. For example, it is a common input stage of op amps, both discrete and monolithic (ICs). In many discrete component designs, resistors are used in place of the constant current sources. However, for the best performance and especially in the design of IC op amps, constant current sinks and sources are deployed. Besides, in IC manufacturing transistors are easier to implement than resistors. Consequently, to the benefit of monolithic op amps, current sources are almost exclusively used.
Figure 3 shows a couple of discrete circuits that can be useful when an op amp would be an overkill. Figure 3a is a complementary voltage amplifier. With the component values shown it has a gain of 40 dB (100), flat from about 0.01 Hz to 3.5 MHz. The circuit Figure 3b is a constant current source supplying 4 mA with the given component values. It could be used as the load in the long-tailed amplifier. Its NPN cousin would be sinking emitter current in that amplifier. With the current mirror, the actual circuit implemented either with discrete components or as a part of a monolithic op amp would look like the circuit in Figure 4.
In Part 2 of this article series, I mentioned that few diodes behave exactly as described by the theoretical equations. The opposite is true about transistors. You can use a transistor collector-base or base-emitter junction as an almost ideal diode. What’s more, a reverse-biased base-emitter junction of a transistor functions as a Zener diode. The popular 2N3904 NPN general purpose transistor, for instance, has a very sharp knee at approximately 6 V, needing just a few microamperes as compared with milliamperes for Zener diodes. This can be useful when power consumption is at premium, as I showed in my past article about infrared detectors (Circuit Cellar 343, February 2019).
A LOOK AT FETS
Now, let’s turn our attention to field effect transistors—commonly called FETs. Interestingly, the invention of a FET preceded the BJT by some two decades. The FET was patented by Julius Edgar Lilienfeld in 1926, but it was never constructed because the necessary technology wasn’t available at that time.
We begin with the JFET structure depicted by Figure 5 with their accompanying schematic symbols. Just as with the BJTs, JFETs come in two flavors: N-channel and P-channel. You can see that the control electrode called gate is a P-N junction forming a diode. To achieve the FET’s signature high input impedance, this diode must be reverse-biased. The result is that the reverse biased gate inhibits the movement of electrons or holes, depending whether the channel is N or P material. The maximum drain-to-source current is obtained at zero bias as shown in Figure 6. For this reason, we say that JFETs are depletion-mode devices.
Similar to BJTs, FET amplifiers can also be created in three basic configurations: Common source, analogous to the common emitter: common gate equivalent to the common base and common drain, also known as a source follower corresponding to the emitter follower. Figure 6 is the drain-to-source current ID versus drain-to-source voltage VDS characteristic with gate-to-source voltage VGS as a parameter. At some negative VGS there will be no ID current flowing. This negative voltage is referred to as the pinch-off voltage VP. In the ohmic region the channel resistance can be calculated based on measured drain-source voltage IDS and drain-source current. Then:
where gm is the FET’s transconductance gain.
Common source configuration is widely used, including front end stages of monolithic op amps where it can be often seen as FET-based long-tailed pairs. It has very high input impedance and good voltage gain. Common gate, analogous to the common base, is not as widely used. But it can be seen as the second stage of a cascode amplifier. Once again, this is analogous to its BJT version with its advantages and disadvantages.
Common drain—in other words, source follower—transforms very high input resistance to a low resistance output. It is frequently an integral part of sensors, such as the passive infrared (PIR). In the saturation region, the saturation current IDSS shows very little dependence on the drain-to-source voltage IDS. This characteristic can be advantageous for the design of constant current sinks and sources.
In a constant current sink (Figure 7), resistor R adjusts the magnitude of the sink current which, for R = 0 is the maximum saturation current IDSS. This, as well as the pinch-off voltage, are device dependent. Using a P-channel JFET in place of the N-channel and switching polarities you will obtain a constant current source.
As you may have deduced from Figure 5, the JFET is a symmetrical device and the drain and source terminals are interchangeable. This makes a JFET useful as an analog switch. And if that wasn’t enough, JFETs have a fairly wide ohmic region where, based on the VGS, they behave as voltage-controlled resistors. This makes JFETs suitable for applications such as volume control, voltage-controlled oscillators, modulators and wherever a variable resistance is needed. Because of its high input impedance and small geometry, FETs found their use in low noise, high frequency circuits with up to 30 GHz frequency limit.
In Part 5 of this article series, we will expand our discussion of FETs.
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CIRCUIT CELLAR • DECEMBER 2019 #353