Lab 6 - The Power Amplifier and LNA - EE133 - Prof. Dutton - Winter 2004 1EE133 - Lab 6RF Amplification: The Power Amplifier and LNA(revisited )1 IntroductionThere will b e no formal Prelab 6 handout. This does not mean that there is no work to be done prior tocoming into lab, however. Please read through this handout to determine what items you must c ompletebefo re coming into lab.In this lab, you will be building and testing a monolithic power amplifier fr om Mini Circuits. This poweramplifier is the last stage of your transmitter before the antenna. This is what will allow you to get muchhigher distance performance out of your FM system. In addition, you will be revisiting your LNA with s omefurther characterization and impe dance matching. We will also look at an important char acteristic of almostany analog circuit, linearity.2 Linearity and The Third-Order Intercept PointAlthough there are multiple ways to measure linear ity, the ones most commonly use d are the third-orderintercept point and the 1-dB compression point. Section 16.5 in the text has a good explanation of thesetwo characteristics.If we can represent the output of an amplifier as a series expansion in terms of the input voltage, then theoutput voltage might be written in this form:Vo(t) = AVi(t) + BVi(t)2+ CVi(t)3+ ...If we assume that 4thand higher order terms won’t cause much error, then we can truncate the serie s afterthe 3rd term. Now, if we assume an input of the following form,Vi(t) = cos (2πf1t) + cos (2πf2t)then the output will have a number of harmonic and what are called inter-modulation terms (that is, termsthat involve mor e than one different frequency). As stated in the book, these terms will appear as follows:• Second harmonics: 2f1, 2f2(from V2iterm)• Third harmonics: 3f1, 3f2(from V3iterm)• Second-order intermodulation products: f1± f2(from V2iterm)• Third-order intermodulation products: 2f1± f2, 2f2± f1(from V3iterm)The third-order intermodulation terms are the ones of concern to us because they appear close to our desiredsignal and be c ause they tend to grow more quickly than the 2ndorder terms as the input power increases.We can test to see how linear an amplifier is by combining two different tones (that is, two signals of differentfrequencies) at the input, and mea suring the levels of the intermodulation products as we increase the inputpower signals. In theory, the third or der terms will increase 3dB for e very 1dB change in input power (On alinear scale, these terms increase as the cube of the input voltage). In reality, however, both the fundamentalfrequency terms and third order intermodulation terms will slope off at high input powers. An example plotof fundamental and third- order power as a function of the input power is shown in Figure 1. The pointat which the output fundamental power fa lls off by 1dB from the e xtrapolated power is known as the 1dBcompression point. Therefore, when you measure IP3, you will have to extrapolate the actual value fromLab 6 - The Power Amplifier and LNA - EE133 - Prof. Dutton - Winter 2004 2values at lower frequencies.Input vs. Output IP3: There are two ways to specify IP3, by referring to the input power (IIP3)or by referring to the output power (OIP3) at which the extrapolated first and third-order power lines in-tersect. Data sheets will often quote OIP3 but list it as simply IP3. This is a marketing ploy to make acomponent look like it ha s better performance than it does, since OIP3 will tend to be a bigger number thanIIP3 (if it weren’t, you wouldn’t have a very good amplifier). Therefore you must be c areful when selectingcomponents based on these specifications.ExtrapolatedIntercept PointActual Output OIP3(ideally)−30 −25 IIP3Input Power (dBm)Slope=1 (dBm)Output PowerP_24.5MHzP_24.5MHzP_24.3MHzP_24.3MHzSlope=3Figure 1: Determining IIP33 The Power Amplifier3.1 Building the Power AmplifierAsk your TA for a power amplifier chip and solder mount boar d. This chip is a surface-mount part, so we’regoing to have to use a slightly different soldering technique to put it on our board. As you can see, the partitself is too small to fit onto our boards directly, so we’ve made up a header bo ard to solder the part to.Once you’ve soldered the surface-mount part on, you can then solder the rest of the circuit to the board.1. The GALI-5: Look at the data page for the GALI-5 (there is a link from the EE133 website) andnote down important specifications for the amplifier. Some things to look for are power gain, frequencyrange, input and output impedances, power consumption, etc.2. Chip Layout and Connection: The chip has three pins and one tab on to p. The middle pin andtab are to be connected to ground, the left pin is the input, and the right pin is the output (this willbe connected to the power supply through an inductor and bias resistor).3. Orienting the Board: Notice that the solder mount board has three plate-through holes, one forthe input and two for the output. This will allow you to connect a co upling capacitor to the input,and it will allow you to connect the RF choke and co upling capacitor to the output. The middle pinand top pad are connected to the small copper ground plane. This ground plane should be connectedto the ground of your circuit. Figure 2 shows the general circuit layout.4. Soldering a Surface Mount Part: Soldering surface-mount parts takes a slightly different approachthan so ldering normal parts. First, without putting the part on the mounting board, place a smallLab 6 - The Power Amplifier and LNA - EE133 - Prof. Dutton - Winter 2004 3From Multiplier MatchingTo Antennaor ColpittsCbyCbyRbiasNetworkCbypassVccGALI−5Lchoke−+Figure 2: MiniCircuits GALI-5 Amplifier Circuitdab of solder on each o f the traces wher e you want the pins to be soldered to the board (this includesthe ground tab and middle pin, which connect to the ground plane on the boa rd). Then place theMiniCircuits part on top of the cold solder. You may want to use a pair of tweezers to hold the partin place. Now heat up the trace with the soldering iron until the solder melts and the pin is securedto the board. Be careful not to overheat the part.5. Building the Rest: Note in Figure 2 tha t the output of the amplifier is connected through a ninductive RF choke (so-ca lled b e c ause it presents a high-impedance to RF signals and so ‘chokes’ themoff) and a bias resistor. This resistor is necessary for the chip to