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KickStart b y M i k e To o l e y Part 1: MOSFET switching devices in linear applications – introducing the 2N7000 ‘Swiss Army Knife’ Our occasional KickStart series aims to show readers how to use readily available low-cost components and devices to solve a wide range of common problems in the shortest possible time. Each of the examples and projects can be completed in no more than a couple of hours using M OSFET devices are available in various forms, including N- t y p e an d P -typ e, an d enhancement or depletion mode types. The 2N7000 is an N-type MOSFET designed for enhancement mode operation (see Fig.1.1). This simply means that to turn the device ‘on’ (ie, to make it conduct) it is necessary to apply a positive voltage between the gate and source of the device. For the 2N7000, the required gate-source voltage is in the range of 2V to 3V. The device will conduct very heavily when the gate-source voltage exceeds about 3V, in which case the resistance between drain and source (RDS) will fall to the very low value required for switching applications. However, with gate-source voltages (VGS) of less than 2.5V the device can be used in a wide variety of linear applications. This is where this incredibly versatile ‘Swiss Army Knife’ finds a whole new variety of applications! Fig.1.2. 2N7000 pin connections. Fig.1.1. Construction of an N-channel MOSFET device. 38 off-the-shelf parts. As well as briefly explaining the underlying principles and technology used, the series will provide you with a variety of representative solutions and examples, along with just enough information to be able to adapt and extend them for their own use. This first part shows you how to use a lowcost MOSFET switching device in a variety of linear applications. In keeping with the KickStart philosophy, we’ve provided sufficient information for you to be able to design and build your own circuits using this handy semiconductor device. MOSFET basics The construction of a typical N-channel MOSFET device is shown in Fig.1.1. The device consists of a series of semiconductor layers onto which an insulating metal-oxide layer is deposited. Conduction takes place between source and drain in a narrow N-type channel. The degree of conduction in this region is dependent on the positive potential Fig.1.3. N-channel MOSFET test circuit. present on the gate terminal. As the gate-source voltage (VGS) is raised beyond the threshold for conduction (usually above about 2V for the 2N7000) conduction increases and the drain and source currents (which are identical) increase as a result. Thus, the voltage present between the gate and source controls the current flowing in the drain. In manufacturers’ data sheets, device properties are usually summarised both in the form of tabulated data and as a series of characteristic Fig.1.4. Output (drain) characteristics for a 2N7000 operating under small-signal conditions. curves. Key data for the 2N7000 is shown in Table 1.1, and series of plots showing the variation the two most important characteristic of drain current (ID) with drain-source curves are shown Fig.1.4 and Fig.1.5. voltage (VDS) for different values of The test circuit that we used to obtain gate-source voltage (VGS). There are a these plots is shown in Fig.1.3. Note few things to note about these curves. that the drain current (ID) needs to be First, we have plotted these curves using relatively small values of drain current kept to a safe value in order to limit that would be used in linear (rather than the total power dissipation of 350mW switching) applications. Second, note quoted in Table 1.1. the bend that occurs for values of VDS The output characteristics (shown in Fig.1.4) consist of a below about 2V. . Practical Electronics | January | 2021 Table 1.1: Key data for the 2N7000 Specification Abbrev. Value Maximum drain-source voltage VDS(max) 60V Maximum drain-gate voltage VDG(max) 60V Maximum gate-source voltage VGS(max) ±20V Maximum drain current ID(max) 200mA Maximum total power dissipation PD(max) 350mW Maximum gate-source threshold voltage VGS(th.max) 0.8V Minimum gate-source threshold voltage VGS(th.min) 3V Minimum forward transconductance gfs 0.1mS Maximum input capacitance Ciss 60pF The transfer characteristics (shown in Fig.1.5) provide us with another family of plots. They show how the drain current (ID) varies with gate-source voltage (VGS) for different values of drain-source voltage (VDS). The three curves shown correspond to VDS values of 3V, 6V and 11V. As can be seen, they are all very similar. It is worth noting that we have plotted these curves using relatively small values of drain current (ID) and, as with the output characteristic curves shown in Fig.1.4, the values of drain current are very much less than those shown in the data sheets published by semiconductor manufacturers. The slope of the transfer characteristic is of particular significance. This is known as the forward transfer conductance (gfs) and is defined as a small change in drain current ( ID) divided by the corresponding small change in gatesource voltage, ( VGS) at a given value of drain-source voltage (VDS), thus: The value of gfs is quoted using the unit of conductance (siemen (S), or even ‘mho’ (which is simply ‘ohm’ backwards!) by some manufacturers). However, since the values are usually fairly small, we use mS (milli-siemen) or mmho instead. Putting this into the context of Fig.1.4, a change in drain current from 6mA to 10.9mA will be produced by a change in gate-source voltage from ΔID 2.3V gfs =to 2.4V.SHence, at this point on the GS transferΔV characteristic when VDS = 6V, we can determine the forward transfer conductance from: gfs = ΔID S ΔVGS gfs = (11 - 6.0 ) mA = 5mA = 50 mS ( 2.4 - 2.3) V 0.1V gfs = (11 - 6.0 ) mA = 5mA = 50 mS ( 2.4 - 2.3) V 0.1V Using MOSFET devices in linear applications As previously mentioned, Fig.1.6 shows the basic components that are used in a simple common-source MOSFET amplifier stage. In order to use a 2N7000 in linear mode it is necessary to provide a gatesource bias voltage of around 2V. The gate bias (Vbias) can be applied to the Fig.1.5 Transfer characteristics for a 2N7000 operating under small-signal conditions. Practical Electronics | January | 2021 Fig.1.6. Bias and load arrangements for a simple common-source MOSFET amplifier stage. gate via a high-value resistor, RB. The value of RB is not critical but is usually in the range 100kΩ to 1MΩ. The output voltage is developed across a load resistor (RL) of suitable value connected in the drain circuit. Capacitors CIN and COUT (respectively) are used to couple the AC signal into and out of the stage. Load lines To understand how the load resistor works it is worth taking a look at Fig.1.7 which shows how a load line can be superimposed on the output characteristics that we met earlier. This particular load line corresponds to a value of 500Ω for RL. The two ends of the load line correspond to the extreme values that would occur when TR1 is either fully conducting (on) or non-conducting (off). The slope of the load line is thus the inverse of RL. The operating point is the point at which corresponding values of VDS and ID will occur when no signal is applied (this is sometimes referred to as the ‘quiescent condition’). When a signal is applied to the stage, VGS will change and the Fig.1.7. Load line superimposed on the 2N7000 output characteristics. 39 Fig.1.8. A basic amplifier using the 2N7000. Fig.1.10. An improved ‘gain block’. half the supply voltage (ie, around 4.5V neglecting the small voltage dropped across R4). Typical operating voltages are shown on Fig.1.8, but in practice, and due to variations in the characteristics of individual components and MOSFET devicFig.1.9. An amplifier stage with a gain of about 40. es, variations of around ±10% can be expected. If significant differences are operating point will move up and down encountered, then the values of R1 and/ the load line. It is then possible to read or R2 can be changed accordingly. off corresponding values of VDS and use The voltage gain of the stage is them to infer the shape of the output approximately 14. This means that a voltage waveform, as shown in Fig.1.7. signal input of 100mVpk-pk will result With the conditions indicated on Fig.1.7, if a 300mV pk-pk signal is in an output of 1.4Vpk-pk. If more gain superimposed on a standing bias voltage is required then a capacitor can be of 2.35V, the value of drain-source voltage introduced in parallel with R4, as shown (VDS) will swing down to a minimum of in Fig.1.9. This capacitor will bypass the signal voltage component that appears 3.2V and up to a maximum of 8.2V. Hence across R4, reducing the amount of negative an input of 350mVpk-pk will produce an feedback and increasing the voltage gain output of 5Vpk-pk corresponding to a as a result. With R4 bypassed to signals, modest voltage gain (AV) of a little over 14. The required bias voltage (2.35V in the case of Fig.1.7) can be obtained in various ways, but one of the most basic arrangements is nothing more than a resistive potential divider derived either from the DC supply rail or from the drain connection. the circuit shown in Fig.1.9 provides a voltage gain of around 40 so that an input of 100mVpk-pk will produce an output of 4Vpk-pk, and so on. The input resistance of both circuits is approximately 500kΩ (determined largely by R1 and R2) and the measured frequency response extends from less than 10Hz to well over 200kHz at the –3dB points. Negative feedback and bias stabilisation The circuit of Fig.1.8 employs two forms of negative feedback: shunt voltage feedback from drain to gate via R3 and series current feedback using R4 in the connection from source to ground. These two feedback loops help to stabilise the DC operating conditions, ensuring that the operating conditions remain within the desired range despite variations in MOSFET parameters and temperature. They also make a significant improvement in the overall linearity of amplifier. Operation of the shunt feedback loop is as follows. If the gate-source voltage increases the drain current will increase as a direct consequence. The increase in drain current will result in an increased potential drop across the load resistance A basic 2N7000 amplifier A basic common-source amplifier is shown in Fig.1.8. The load for the stage is provided by R3, while the gate-source bias voltage (which needs to be approximately 2.2V for optimum operating conditions in this circuit) is defined by the potential divider formed by R1 and R2. Optimum operating conditions are those that will give the maximum undistorted output voltage swing. This condition occurs when the drain voltage is about 40 Fig.1.11. Using Virtins Multi-Instrument PC-based software to analyse and measure the total harmonic distortion (THD) produced by the circuit of Fig.1.10. Practical Electronics | January | 2021 on the drain load of the first stage (TR1) and produces a nominal voltage gain of 10. The measured frequency response extends from around 4.5Hz to 450kHz. Notice how negative feedback (via R1) is applied over both stages (ie, from the source of the second stage to the gate of the first stage). Distortion Fig.1.12. Using Virtins Multi-Instrument PC-based software to analyse and measure the total harmonic distortion (THD) produced by the circuit of Fig.1.10. No amplifier is perfectly linear, and no amplifier can provide a perfect representation of its input. The result of non-linearity is distortion and, while a small level of distortion may be undetectable by ear, designers usually go to great lengths to reduce the level of distortion introduced by an amplifier. It is important to note that, as an amplifier is driven harder, the distortion that it produces will increase. The circuit shown in Fig.1.10 Fig.1.13. A twin-tee oscillator based on the two-stage gain block in Fig.1.10. Fig.1.15. 8MHz crystal oscillator using a 2N7000. (RL). As a result, the drain voltage will fall, which, in turn, reduces the gate bias voltage. Capacitors, C1 and C2 respectively, are used to couple signals into and out of the stage. Put simply, these capacitors pass AC signals but keep the DC gate bias and drain voltages safely within the amplifier stage. An improved ‘gain block’ An improved two-stage amplifier is shown in Fig.1.10. The first stage acts as a common-source amplifier with a high input impedance; the second as a sourcefollower with a low output impedance. The two-stage ‘gain block’ arrangement reduces the effect of output loading typically produces around 0.25% total harmonic distortion (THD) when a signal of 100mVpk-pk is applied, as shown in Fig.1.11. (Note that the THD increases to around 6% for the circuit of Fig.1.9 with the same signal.) It is therefore important to avoid overdriving an amplifier, particularly where the internal gain is appreciable. A high-gain amplifier A high-gain amplifier is shown in Fig.1.12. This arrangement uses two cascaded common-source amplifier stages. This circuit produces a typical voltage gain of 250 with C3 not fitted, increasing to around 370 with C3 fitted (as shown in Fig.1.12). Note that signal levels must be kept low to avoid overdriving the second stage. Sinusoidal oscillators Fig.1.14. Using Tina Pro to check and optimise component values for the twin-tee oscillator shown in Fig.1.13. Practical Electronics | January | 2021 As well as its use in small-signal amplifiers, the 2N7000 can be used in a wide variety of sinusoidal oscillator applications. Fig.1.13 shows a simple twin-tee oscillator based on the twostage gain block in Fig.1.10. The circuit provides an output of around 8Vpk-pk at approximately 100Hz. The frequency of 41 Table 1.2: Going Further with the 2N7000 MOSFET Topic operation can be altered by varying the values used in the twin-tee network (see Going further below). Note that popular Spice-based simulation software can often be used to model and optimise the operation of MOSFET circuits, as shown in Fig.1.14. Finally, Fig.1.15 shows a crystalcontrolled oscillator based on a single 2N7000 device. This provides a typical output of 2Vpk-pk and has been tested with quartz crystals between 2MHz and 20MHz. This arrangement forms the basis of the author’s own simple Crystal Checker circuit, which he uses in his workshop. Going further Our Going further table (opposite) will help you locate the component parts and further information that will allow you to quickly progress with your own designs and modifications. It also provides you with background reading that will help you get up to speed with the necessary underpinning knowledge for key topics discussed. GET T LATES HE T COP Y OF TEACH OUR -IN SE RIES AVAIL AB NOW! LE Source Notes 2N7000 MOSFET The 2N7000 is available from many suppliers, including CPC/Farnell, Mouser, RS Components and numerous online suppliers. Data sheets can be downloaded from manufacturers’ websites including ON Semiconductor, Fairchild and Vishay Prices range from a few pence to about 50p depending on the quantity purchased Audio amplifiers For all your audio amplifier requirements, the PE column Audio Out provides a treasure chest of tips, hints, designs and ideas, drawing on Jake Rothman’s decades of experience in teaching, designing and building a huge range of audio circuits. All Audio Out columns are available via back issues at: www.electronpublishing. com Circuit simulation The author’s own book, Electronic Circuits: Fundamentals and Applications (Fifth Edition 2020 published by Routledge 9780367421984) provides an introduction to circuit simulation based on the popular SPICE-based Tina Pro software Tina software can be obtained from www.tina. com. A cut-down version can be freely downloaded from Texas Instruments at: www.ti.com/tool/TINA-TI Distortion A detailed explanation of different types of distortion can be found in Part 8 of Electronics Teach-In 7 www.electronpublishing. com/product/electronicsteach-in-7 Negative feedback For a useful introduction and relevant theory see Part 8 of Electronics Teach-In 7 www.electronpublishing. com/product/electronicsteach-in-7 Transistor characteristics and load lines The author’s own book, Electronic Circuits: Fundamentals and Applications (see above) provides an introduction to transistor characteristics, load lines and amplifiers. Order direct from Electron Publishing PRICE £8.99 (includes P&P to UK if ordered direct from us) EE FR -ROM CD ELECTRONICS TEACH-IN 9 £8.99 FROM THE PUBLISHERS OF GET TESTING! Electronic test equipment and measuring techniques, plus eight projects to build FREE CD-ROM TWO TEACH -INs FOR THE PRICE OF ONE • Multimeters and a multimeter checker • Oscilloscopes plus a scope calibrator • AC Millivoltmeters with a range extender • Digital measurements plus a logic probe • Frequency measurements and a signal generator • Component measurements plus a semiconductor junction tester PIC n’ Mix Including Practical Digital Signal Processing PLUS... YOUR GUIDE TO THE BBC MICROBIT Teach-In 9 Teach-In 9 – Get Testing! A LOW-COST ARM-BASED SINGLE-BOARD COMPUTER Get Testing Three Microchip PICkit 4 Debugger Guides Files for: PIC n’ Mix PLUS Teach-In 2 -Using PIC Microcontrollers. In PDF format This series of articles provides a broad-based introduction to choosing and using a wide range of test gear, how to get the best out of each item and the pitfalls to avoid. It provides hints and tips on using, and – just as importantly – interpreting the results that you get. The series deals with familiar test gear as well as equipment designed for more specialised applications. The articles have been designed to have the broadest possible appeal and are applicable to all branches of electronics. The series crosses the boundaries of analogue and digital electronics with applications that span the full range of electronics – from a single-stage transistor amplifier to the most sophisticated microcontroller system. There really is something for everyone! Each part includes a simple but useful practical test gear project that will build into a handy gadget that will either extend the features, ranges and usability of an existing item of test equipment or that will serve as a stand-alone instrument. We’ve kept the cost of these projects as low as possible, and most of them can be built for less than £10 (including components, enclosure and circuit board). © 2018 Wimborne Publishing Ltd. www.epemag.com Teach In 9 Cover.indd 1 01/08/2018 19:56 FREE COVER-MOUNTED CD-ROM On the free cover-mounted CD-ROM you will find the software for the PIC n’ Mix series of articles. Plus the full Teach-In 2 book – Using PIC Microcontrollers – A practical introduction – in PDF format. Also included are Microchip’s MPLAB ICD 4 In-Circuit Debugger User’s Guide; MPLAB PICkit 4 In-Circuit Debugger Quick Start Guide; and MPLAB PICkit4 Debugger User’s Guide. ORDER YOUR COPY TODAY JUST CALL 01202 880299 OR VISIT www.electronpublishing.com 42 Practical Electronics | January | 2021