Labratry Design f an Instrumentatin Amplifier OBJECTIVE T esign an implement an p-amp base instrumentatin amplifier. BACKGOUND An instrumentatin amplifier is typically the first stage in an instrumentatin system. It is use t amplify the signal pruce by a transucer such as a thermcuple r a strain gauge. An instrumentatin amplifier is a ifference amplifier i.e., it amplifies the ltage ifference between its tw input terminals, neither f which is require t be a signal grun. An instrumentatin amplifier shul hae the fllwing characteristics: high input resistance, high ltage gain, an high mn-me-rejectin-rati (CM). Cmmn-me signals an CM A mn-me signal is a signal that is mn t bth input terminals f an amplifier. In an instrumentatin system, the wires leaing frm the transucer t the amplifier can pick up mn me interference. The mn-me signal can be f the rer f a few lts, while the ifference signal t be amplifie can be much smaller e.g., in the millilt range. Instrumentatin amplifiers must be able t amplify the esirable ifference signal (ente ) an reject the unesirable mn-me signal (ente ). The CM is a figure f merit f an amplifier that specifies hw g it is at rejecting mn me signals. Let A be the ifferential ltage gain f the amplifier an A be the mn-me ltage gain. The CM is efine as fllws: CM = A / A. Fig. -: Cmmn-me an ifference me signals Cnsier an p-amp perating pen-lp as epicte in Fig. -. The ifference signal is = p - n, an the mn-me signal is = ( p + n )/. In the case f Fig. -, the ifference signal is = 0, an the mn me signal is = V. If the p-amp is ieal, the utput signal is = A (nly the ifference signal appears at the utput). The utput signal is thus = 0 if
the p-amp is ieal. Hweer the mn me signal es prpagate t the utput if the pamp is nn-ieal. Fr a nn-ieal p-amp, the utput signal is = A + A Eq. - The typical ltage gain A f an LM 74 p-amp is 00,000 an the typical CM is 90B. Nte that the CM in B is just 0 lg ( A / A ). The cnfiguratin f a basic ifference amplifier is epicte in Fig. -. Using ieal p-amp analysis it is easy t shw that the input resistance f the amplifier is an that the utput signal f the amplifier is = / ( ). The amplifier thus has zer mn-me gain an a ifferential ltage gain f /. Nte that the mn-me gain is nt zer if the pamp is nn-ieal, r if the resistrs labele (r ) nt hae ientical alues. Let p an n ente the ptentials at the + an - terminals f the p-amp f Fig. -, respectiely. Let us nw analyze the basic ifference amplifier assuming that the p-amp is nnieal. Specifically, let us assume that the p-amp has finite gain A (resulting in p - n being nnzer). We will als assume that the p-amp has infinite input resistance (n current flws int the + r - terminal). Let an ente the ifference an mn-me signals f the ifference amplifier, f Fig. -, respectiely. We hae + = an = Eq. - Similarly, let an ente the ifference an mn-me signals at the p-amp input. We hae It is clear frm the circuit iagram that Fig. -: A ifference amplifier p + n = p n an = Eq. -3 = p + Eq. -4
n = + ( ) = + Eq. -5 + + + The utput signal expresse in terms f the p-amp ifferential gain A an mn-me gain A is = A + A Eq. -6 Using Eqs. -3, -4, an -5 in Eq. -6 yiels the fllwing expressin fr : A A = ( ) + ( / ) Eq. -7 + + + Cllecting all the terms cntaining we btain the fllwing expressin fr (in terms f an f Eq. -): A + A = Eq. -8 A + + A The enminatr f the abe equatin is apprximately equal t A since A >> A an A >> / (the gain f the ieal ifference amplifier). We thus hae + Eq. -9 CM where CM = A / A is the mn-me rejectin rati f the p-amp. It is clear frm the abe equatin that the ifferential ltage gain A -amp an mn me gain A -amp f the amplifier are as escribe belw: A amp = A amp = Eq. -0 CM The mn-me rejectin rati f the ifference amplifier is the same as that f the p-amp, as emnstrate belw: CM = amp = A amp / A amp CM Eq. - As mentine earlier, the mn me signal can be rers f magnitue higher than the ifference signal: the mn-me pnent f Eq. -9 can thus be significant. It is clear frm the abe iscussin that the basic ifference amplifier suffers frm tw isaantages: lw input resistance, an inaequate mn-me rejectin. The instrumentatin amplifier The instrumentatin amplifier epicte in Fig. -3 es nt suffer frm the isaantages liste abe; it has high input resistance an high CM. It is clear frm the circuit iagram that the input resistance seen by the surce is gerne by the input resistance f the p-amps use in the circuit. The input resistance f the instrumentatin amplifier is thus ery high.
Figure -3: An instrumentatin amplifier The instrumentatin amplifier cnsists f an input stage fllwe by a secn stage (which is just a basic ifference amplifier). It is easily shwn that the ifferential ltage gain f the first stage is ( + / ). We knw that the ifferential gain f the secn stage is 4 / 3. The erall ifferential gain f the instrumentatin amplifier is thus 4 = + Eq. - 3 A amp It is easily shwn that the mn-me ltage gain f the first stage is unity. We knw (see Eq. -0) that the mn me gain f the secn stage is 4 /( 3 CM ). The erall mn-me gain f the instrumentatin amplifier is thus A 4 amp = Eq. -3 CM 3 The rati f Eqs. - an -3 gies the CM f the instrumentatin amplifier. We hae CM + Eq. -4 amp = CM The CM f the instrumentatin amplifier is thus greater than that f the p-amps by a factr ( + / ) which can be large. In fact, if we set 4 = 3, we see frm Eq. - that this multiplying factr is the (large) ifferential ltage gain f the instrumentatin amplifier.
PELAB. The typical ltage gain A f an LM 74 p-amp is 00,000 an the typical CM is 90B. Use these alues t calculate the mn me gain A f the 74 p-amp.. Pre that the ifferential ltage gain f the input stage f the instrumentatin amplifier is ( + / ) 3. Pre that the mn-me ltage gain f the input stage f the instrumentatin amplifier is unity. 4. Design an instrumentatin amplifier that has a ifferential ltage gain f 50. 5. Simulate yur instrumentatin amplifier using PSpice (use ua 74 p-amps) an measure the fllwing parameters. Als pare yur measurements with alues preicte by thery. a) Differential ltage gain f the instrumentatin amplifier b) Cmmn-me ltage gain f the instrumentatin amplifier c) Input resistance f the instrumentatin amplifier IN LAB. Use the circuit f Fig. -4 t prie the ifferential input t the instrumentatin amplifier. What is the theretical alue f the ifferential ltage = -? Measure the ifferential input ltage. Why can't yu iew the ifferential input signal n the scillscpe using a single scpe prbe? Fig. -4: Circuit t generate test input signals. Cnstruct the instrumentatin amplifier that yu esigne in the prelab. Test the amplifier using the input signals frm the circuit f Fig. -4. Measure the ifferential gain f the amplifier. Cmpare with the expecte amplifier perfrmance.