Lab 6 The Power Amplifier and LNA EE133 Prof Dutton Winter 2004 1 EE133 Lab 6 RF Amplification The Power Amplifier and LNA revisited 1 Introduction There will be no formal Prelab 6 handout This does not mean that there is no work to be done prior to coming into lab however Please read through this handout to determine what items you must complete before coming into lab In this lab you will be building and testing a monolithic power amplifier from Mini Circuits This power amplifier is the last stage of your transmitter before the antenna This is what will allow you to get much higher distance performance out of your FM system In addition you will be revisiting your LNA with some further characterization and impedance matching We will also look at an important characteristic of almost any analog circuit linearity 2 Linearity and The Third Order Intercept Point Although there are multiple ways to measure linearity the ones most commonly used are the third order intercept point and the 1 dB compression point Section 16 5 in the text has a good explanation of these two characteristics If we can represent the output of an amplifier as a series expansion in terms of the input voltage then the output voltage might be written in this form Vo t AVi t BVi t 2 CVi t 3 If we assume that 4th and higher order terms won t cause much error then we can truncate the series after the 3rd term Now if we assume an input of the following form Vi t cos 2 f1 t cos 2 f2 t then the output will have a number of harmonic and what are called inter modulation terms that is terms that involve more than one different frequency As stated in the book these terms will appear as follows Second harmonics 2f1 2f2 from Vi2 term Third harmonics 3f1 3f2 from Vi3 term Second order intermodulation products f1 f2 from Vi2 term Third order intermodulation products 2f1 f2 2f2 f1 from Vi3 term The third order intermodulation terms are the ones of concern to us because they appear close to our desired signal and because they tend to grow more quickly than the 2nd order 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 different frequencies at the input and measuring the levels of the intermodulation products as we increase the input power signals In theory the third order terms will increase 3dB for every 1dB change in input power On a linear scale these terms increase as the cube of the input voltage In reality however both the fundamental frequency terms and third order intermodulation terms will slope off at high input powers An example plot of fundamental and third order power as a function of the input power is shown in Figure 1 The point at which the output fundamental power falls off by 1dB from the extrapolated power is known as the 1dB compression point Therefore when you measure IP3 you will have to extrapolate the actual value from 2 Lab 6 The Power Amplifier and LNA EE133 Prof Dutton Winter 2004 values 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 intersect Data sheets will often quote OIP3 but list it as simply IP3 This is a marketing ploy to make a component look like it has better performance than it does since OIP3 will tend to be a bigger number than IIP3 if it weren t you wouldn t have a very good amplifier Therefore you must be careful when selecting components based on these specifications Output Power dBm Actual Output Slope 1 OIP3 Extrapolated Intercept Point P 24 5MHz P 24 5MHz P 24 3MHz Slope 3 ideally P 24 3MHz 30 25 IIP3 Input Power dBm Figure 1 Determining IIP3 3 The Power Amplifier 3 1 Building the Power Amplifier Ask your TA for a power amplifier chip and solder mount board This chip is a surface mount part so we re going to have to use a slightly different soldering technique to put it on our board As you can see the part itself is too small to fit onto our boards directly so we ve made up a header board 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 and note down important specifications for the amplifier Some things to look for are power gain frequency range input and output impedances power consumption etc 2 Chip Layout and Connection The chip has three pins and one tab on top The middle pin and tab are to be connected to ground the left pin is the input and the right pin is the output this will be 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 for the input and two for the output This will allow you to connect a coupling capacitor to the input and it will allow you to connect the RF choke and coupling capacitor to the output The middle pin and top pad are connected to the small copper ground plane This ground plane should be connected to 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 approach than soldering normal parts First without putting the part on the mounting board place a small 3 Lab 6 The Power Amplifier and LNA EE133 Prof Dutton Winter 2004 Vcc Rbias Cbypass Lchoke From Multiplier Matching Network GALI 5 or Colpitts Cby To Antenna Cby Figure 2 MiniCircuits GALI 5 Amplifier Circuit dab of solder on each of the traces where you want the pins to be soldered to the board this includes the ground tab and middle pin which connect to the ground plane on the board Then place the MiniCircuits part on top of the cold solder You may want to use a pair of tweezers to hold the part in place Now heat up the trace with the soldering iron until the solder melts and the pin is secured to the board Be careful not to overheat the part 5 Building the Rest Note in Figure 2 that the output of the amplifier is connected through an inductive RF choke so called because it presents a high impedance to RF signals and so chokes them off and a bias resistor This resistor is necessary for the chip to operate correctly Do not connect the output of the chip to power without this bias resistor The value of this resistor should be about 60 and should …