FET Amplifiers PDF

# FET ## UNIT-## **Dr. E. Kumaraswamy.** Field Effect Transistor (FET) amplifiers provide an excellent voltage gain with added feature of a high input impedance. They also lowpower-consumption configurations with good frequency range and minimal size and weight. The depletion MOSFET Circuit has a...

# FET ## UNIT-## **Dr. E. Kumaraswamy.** Field Effect Transistor (FET) amplifiers provide an excellent voltage gain with added feature of a high input impedance. They also lowpower-consumption configurations with good frequency range and minimal size and weight. The depletion MOSFET Circuit has a much higher input impedance than a similar JFET Configuration. ## BJT 1. BJT device controls a large output (collector) current by a small input (base) current. 2. BJT is a Current Controlled device. 3. Compared to FET, it has low input impedance. 4. BJT has an amplification factor, B (beta). 5. The BJT Can be used as a linear amplifier or a digital device is Logic circuits. Also widely used is high frequency applications. ## FET 1. FET device controls an output (drain) Current by a small input (gate) voltage. 2. FET is a Voltage Controlled device. 3. FET has high input Impedance. 4. FET has a transconductance factor, Im. 5. The FET Can be used as a amplifier or a 6. Voltage gain of FET amplifier is generally less than that obtained using a BJT amplifier. 7. FET amplifier provides much higher input impedance than that of pe BJT configuration. 8. Output impedancea Values are Comparaste fu 6015 BJT and FET Circuits. 9. There are mpn and pnp bipolar transistors, there are n-channel and pichannel field effect transistors. 10. BJT is a biploar device - bi indicates that the Conduction is a frünction of two charge carriers, electrons f and holes. 11. FET is a Unipolar device. depending solely on either electron (n-channel) or hole (p-channel) Conduction 12. FET are more temperature stable than BITS. ## Types: FET are three types. - Junction field Effect Transistor. (JFET). - Metal-oxide- semiconductor field effect tramcision (MOSFET) - Metal-semiconductor field effect transiston (MESFET). The ASOSFET JFETS are 2-types - n-chamel FET - P-Channel FET. the MOSFETS ane 2-types - Depletion MOSFET - Enhancement MOSFET! The MOSFET transistor has become one of the most imported devices used in the design and Construction of Integrater Circuits for digital Computers. ## FET - JFET or FET (Junction Field Effect Transistor ) - MOSFET of IGFET. (Insulated Gate FET (00) metal oxide Semiconduein (MC transistor (MOST or MOSFET - P-Channel FET - n-channel FET - Depletion MOSFET - Enhancement MOSFET. ## Construction and characteristics of FET The basic construction of the m-channel JFET is shown below. - Drain (D) - The major part of the structure is the n-type material, which forms the channel between the embedded layers of p-type material. - Source(s). - The top of n-type channel is Connected through the omic Contact to a termind Called Drain CD., where as the lower end of same material Connected through the omic contact to a terminal Called source (s). - Gate (G). - There are two layers of p-type materids are connected together Halled "Gate (G)." - Drain - Babater analogy fr JFET Control mecham sm - Gate - The result is a depletion region at gach junction, as shown channel. ohmic contact. Depletion region. ## Vgs=0 VDs some positive Valve The current carriers hav diffused across the junction, kaving only uncovered positi ions on the n-side and negative ions on the p-side. The electric limes off field intensity which now originate are precisely the voltage drop across the junction. As the reverse bias across the junction is creases, also does the thickness of the region of immobile uncovered charge the conductivity of this region ism. Hence we see that the effective width of channel will become progressively decrease with increasing reverse bias. Accordingly, for a fixed Veggs, the da draus to source is a function off thereves! Voltage, the drain current will be the drain current will be a function off the reves! biasing voltage across the gate junction. The term Field Effect is used to describe this device because the mechanism of current Control is the effect of the extension, with increasing reverse bias, of the field) associated wiit The region of uncovered changes. ## When Vgs= 0 & Vds=0 - no Voltage is applied between and Source hence Ia = 0. - when Vgs=0 & Vas, increases i-e, majority Carsiess flow thorough channel from Source to dorain. - Id also increases. - The magnitude of the current will depends on no. of majority Casiers (es) available off length of channel and magnitude of applied voltage Vefs. - when Vgs is (decreased from zero) is is creased in-e, reverse bias Voltage is increased across the junction. - hence the channel width is decreased. (Due to thickness of depletion region wider). - Ar certains Voltage, the cross sectiorid area (channel widit) is Constant. - At this, (Vds=Vp), the channel is said to be pinch off, draum vo Hage (Vas) is called (Up)" pinch off votter ## The FET small signal model The linear Small- Signal equivalent circuir for the FET Can be Obtained in a manner analogous to that used to derive the Corresponding model for a transistor. We Can express the drain current is as a function of the gate Voltage VGs and drain voltage Vos by iD = f (VGS, VDS). In the small signal notation. 1x = gm Vgs + + Vas. where gm = d/d Vgs Vds is the mutual conductance Cor trams conductance. (It is dso designated by Yfs or gifs and called forward tram admittance.) The second parameter Yd is the draus (02) Output resistance, and is defined by Yd = d id Vgs ..... The reciprocal of of is the drain conductance 88. (It is dso designated by Yos and gos and called the Output Conductance). ## An amplification factor M for an FE'! M = d ids Vgs | U=0. we can verify that M, and and are related by M =rjgm from draus current is expression, by setting id=0 is = gings + Ves ## FET model: Gate+ ↑ Sources Vgs C On Drais D そ d Imgs C S Fig: The low frequency small signal FET model This low frequency small signal model has a Norton's output circuit with a dependent current generator whose current is proportiond to gate to source Voltage. The cccpeet output resistance is rd., which i Consistent with the definition in a = vas d Ngs. The imput resistance between gate and Sisurce is infinite, Since it is assumed that the reverse biased gate takes no Current. For the same reason the resistance between gate and draus is assumed to be unfinite. ## Insulated Gate FET (IGFET) or MOSFET The nochannel MOSFET Consists of a lightly p-type Substri into which two highly doped nt regions are diffused.. These it sections, which will act as Source and Drain - metal S G D - A thin layer of insulating sioz (silicon dioxide) layer, allowing contact with the Source and drain. - S - nt - G - nt - D -nt - PCsubstrate) The metal area of the gate, in conjunction with the insulating dielectric oxide Layer and the semiconductor channel, forms o parallel plate capacitor. The insulating layer of silicon dioxide is the reason why this device is Called " Insulated Gate FET, This layer result in an extremely high usput resistance (1010 to 1016 2) for mosfet. ## Enhancement MOSFET If we ground the substrate of above figuse and apply a positive voltage at the gate, an electric field will se end on the semiconductor Side. directed perpendicularly through the oxide. This field will "induced negative charges The negative charge of electrons which are minorit, carriers is the P-type substrate forms an Inversion layer! As the positive voltage on the gate increases, the induced mesative charge is the semiconductor increases. The region below the oxide has now has n-type Carriers; the conductivity increases, and current flows from Source to drain through the induced channel. Thus the drain current is " enhanced by the positive gate voltage, and such a device is called an" enhancement type Mosfel - S G D - ht Induced channel nt P (substrate). ## Depletion MOSFET An n-channel is diffused between the source and the drain. - S G D - nt - P(Substrate) Fig@Depletion type mOSFET With this device an appreciable draus Current Ioss flows foo for zero gate tho source Voltage, Ugs=0. If the gate voltage is made negative, positive charges are induced is the channel through thesion of the gate capacitor. For Since the current is FET is due to majority carriess Les for an ntispe materials )7,, The induced stan positive charges make the channel Less conductive ) and the drais current decreases, as ugs is made more negative The redistribution of charge in the channel Causes an effective depletion of majority carriers, which accounte for the designation i Depletion MOSFET' A MOSFET of depletion spe may also operated ws an enhancement mode. If the gate voltage is made positive, the negative charges are induced into the n-channel. In this manner Conductivity of the channel increases and current rises above Ipss. - Idma - enhancement mode - <Depletion region - Depletion mode - enhancement region The discussion is applicable in principle also to the p-channel MOSFET, for such a device the signats signs of all currents and voltages is the V-I Characteristics must be reversed. Most mosfets are triodes, with the substrates internally Connected to the source. - Symbols of n-channel mosFET - D SS G S a G i S D S - enhancement MOSFET - @& either enhancement of depletion type. - Symbols of n-channel PET D G ## FET Amplifiers: JFET provide an excellent Voltage gain with added advantages of high is put impedance There are three basic FET Configuration (similar to BJT). - Common Source CCS) - Common Drain (CD) - Common Gate (CG), ## Common Source amplifier wilk fixed bias! FET amplifier circuit ase the voltage controlled nature of the JFET. In the pinch off region Id depends only on Vgs. (approximately). - RG +b S - C2 - ID - V - DSA - Fig. Common Source FET circuit. On the output characteristics, (Jdmay10) V. I characteristic f an Road liner (Vo) and (demo) FET in biased at Q-point (VDsQ, IDQ1). 2-point is selected approximately at the midpoint of loadlin ## Common Source amplifier Analysis: The Coupling capacitors C1 and (2 which are used to isolate the de biasing from the applied ac signal. - Common Source amplifier circuit equivalent cas follows - All Capacitors and de supply voltage with short circuit - JFET with its low frequency equivalent Circuit. - RD - Vo - Vo - Vo - G - ↑ - f - RD Vas - D - Vit - Co - RG di - RG - Input impedance. - Z=RG - Fig: equivalent circuit (model) for the Common Sour ce amplifier circuit. - Output impedance: Zo is measured looking from the output side with input voltage vi equal to 0. - 1. Vgs=0 - • - = Rollrd. - fra>> Ro - ≈RD - Voltage gais (Av): - from circuit, -gmugs. (rall Rom). - Av=v=-gm (rall Ro). - Vds - Av= - Vo - Vi - ugs - = - AVN-JORD - V=Vgs - it raspro. ## Common Source Amplifier wilk voltage Divider bias; - +DD - C2 - Hez No - G - G - RS - ↑ - Vi - RD - Vgs - Jongs - Fig: Common Source JFET amplifier wilt voltage divider bias. - fig: equivalent circüt. - Here. - G - 2:= .RG = Rill R2. - 20=rall Ro - Av=-gm (rallRD). ## Common Drain (Source Follower) Amplifier: In Common drain amplifier Circuit, is put is applied between gate and source, and output is taken between Source and drais. - VOD - DD - D - G - Vo - CI - and source, - S - RG - - RS - - Vo - Fig: Common drain Amplizie - Simaptified equivalent catrenit - ↑ - V - 9 - D - gings - S - RS - Cz - S - V - No - RS - Simplified equivalent anoded in Common drain circuit - S - G - ↑ - R - Imgs. - Zi-Ri - ZFho - U。 - input impedance (z:): - Zi = RG - Output Impedance (zo): - 7. = rall Rs - Av=Vo - V - Voltage gain (Av): - the output, - V = Is (rallies) - V= gm Vgs (rall Rs). - In common dran, Vì = Vgstvo = Vgs + Shoygs (Talls). - Hence source - hence - Imgs - Au= Vo = Imgs (7a 110%) - Vi - =s-gm (alles) - Ugs + graugs (ralles) Ygs (it gin (rall!) - Av om (mall Ps) - 4 - Αντι - 1+gm(rallRs) - ie, Vovi - (olp) voltage follows always usput voltage, - hence Common drain amplifiler called as 4 Bource follower. ## Common Gate Amplifier: - VOD - V - - RD - - o/p - - - VS - - - - - Input Resistance (Ri) - Input resistance R is measured between source and ground and is gries by - Ri= - output resistance (RO) - Rò = RellIRD = Pra+(4+1DRE JHORD. - Volted gain (Av) = (amra) Rd - Since Av in tve, there is no phase sel betureen iminut && output. ## Generalized FET amplifiers The Circuit Contains three îndependent signal sources, Ve în series with gaite, Vs in series wilt Source and Va in senes with drous. - V= = Va=0 - the input signal is Ve and the output is Voi taken at the draws. - For CG amplifier - Vi = Va=0, - the impur signd is Ve wilt Soume Resistance Rs and output is Vor taken at the draên. - for CD amplifier (source follower) - Ns=Va=0, - Ra=0. - the input signal in Vo and the output is Voz taken at the Sousel ## Common Source Amplifier wilk Resistive Loadi - Rs should be much less than the amplifier input resistance, - Ri = Roll R₂ worder to minimize loading effects. - the input voltage Ugs is givent - Ugs = - ⇒ V = Vgs - the output voltage Vo iês given by - Vo = -gunvigs (rall rallire). - ... Voltage gaius Av = - Gonamon - (2) - Vo - Vi - = - R - 5 - +VDD - , - Vo - D - C2 - RL - Fig: cs amplifier wit (Rit!) Resistive load (Voltage divider bias) - -8mgs/(rall Rall Re). - Av = " -9m(Ritri) (rallrall Re) - Ri - +VOD - Ygs - (Ritle) - Ri - - : - output - D - input - S - fig: cs amplifier wilk Resistive Load, ## Common Source Amplifier with Diode Connected Load: - IT is difficult to fabricate accurate Values of resistor with a reasonable physical size. Hence it becomes practical to replace Ro with a mos transistor. - We can operate mos transistor as a small signal resistor by shorting its gate and drain terminals as shown in fig - The resistance, i prorsided by the diode Connected load is much higher than a resistance RD and hence mos amplifier with diode Connected load provides higher vo Hage gaiss. ## MOSFETs as Current sources - A MOSFET behaves as a Current Source when it operating is the Saturation region (pinch offregion). - An NMOSFET: draws current from a point to ground (sines curcent) whereas PMOSFET draws cwrent from a VDD to a print (Sources cured for fixed Vas. - X acgnd. @Nmos x - ← - $ - Id: - VOD - -Vgs=const. VoK 어서 - vas - ac gnd. - mos.Y - - Y - VOD - It Vas Vary, with Ux and Vy. they Do not behave like Current Sources ## Common Source Amplifier with current source Lond - we know that the Vollege gain fr es mos. amplifier is given by - Av=-gin Ro. - Thus to have large voltage gain, - we need to have high Value of RD... - we have seen that, we Can provide large value of Ro by using diode connected Load. - However, iscreasing load resistance Value will limit the Output voltage sương. This problem canse solved by using Current source load instead of diode Connected Load. - → It can be seen from the equivalent circuit, that - V - =-gmı (roll roz) Vgsi - ∴ Av = = a(roll road - .. Vo - Vi - G 2 - N - DD - M2 - Vi - MI - Fig: Common Source amplifier with Current Source Load. ## MOSFET source follower Amplifier: - Nmos source follower shown infig. - the output is taken from the Source with crespect to ground and drain is com Connected directly to VDD. - Vgsi=Vi. - NDD - RR - C2 - Vo - - ات و - Fig: Common Drais Amplifie (Sorisce follower) ## Cas Code Amplifier - The cas code amplifier Consists of two mos transistors as shown in big. - Cascoding refers to the use of a transistor connected us the Common-gate configuration 1) - Provide current buffering for the off of Commo tran - M - Fig: Cascode amplifier of common soursee arising ## Folded Cascode Amplifier:. - The current Di is determined by the usput voltage... - The de current in m₂ is the difference between the bias Curent Is and I.. - Here, the ac Current in through bole transistors and ground. - The ac current is M₂ is equal in magnitude but is the opposite direction to MI. Thus the current is sand to be folded back and the circuit is Calld" folded cascode Circuit' - HUDD. - гта - fig: Folded Cascade Amplifier ## Frequency Response of Mos Cascode Amplifier: - Ultimate Bandwidth of mos amplifier is determined by Speed" limitations of the MOSFET itself, this means that it takes Certain time for charge carriers to move from Source to drain. - The units gain frequency (fr) is given bη - (fr) - fr≈ - Im - 211 (Cgs+ Cgd). - where - gm is trans conductance. - Cgs and Cgd are MOSFET Capacitances, - Gain (dB) - low. / mid Band frequenais & high frequency band, - Land - Gaus fall - =-3dB-due to the effect" of - Gain falls of - dul to the effect of Cgs, Cod. - 0 - fi - fH - frequency. - Fig: Frequency Response. of mas amplifiers. - where - from (low caroth & - Σπ frequency - fn. 2TH C₂(Rell Rollos). - (High cutoff frepciency).