Lecture 8. RF Amplifier Design. Johan Wernehag Electrical and Information Technology. Johan Wernehag, EIT

Transcription

1 Lecture 8 RF Amplifier Design Johan Wernehag Electrical and nformation Technology

2 Lecture 8 Amplifier Design Summary of Design Methods Transistor Biasing Voltage and Current Drive of Bipolar Transistors Temperature Dependence Passive Biasing Circuits Active Biasing Circuits Biasing of Field Effect Transistors solating the Bias Design from Signal Designs Diagnostic test

3 Summary of Amplifier Design Methods Specific G T and F 1. Decide if the transistor is unconditionally stable or not 2. Calculate stability circles if the transistor is conditionally stable Γ S F 3. Choose the design method for specific gain 4. Assume that conjugate match will be applied at the output 5. Calculate and draw the gain circle (g a < G MSG ) and noise circle (F) Stable area Γ S g a Γ L Γ OUT Stable area Γ L 6. Select a Γ S in the stable area that provides a suitable compromise between gain and noise figure S 11 S Calculate Γ OUT regarding the selected Γ S 8. Calculate Γ L = Γ OUT * and check for stability 9. Design the matching networks and verify stability for all frequencies of interest 10.Design the biasing circuits for proper DC settings of the transistor

4 Transistor Biasing How is the bias setting specified? collector current and collector-emitter voltage V CE for BJT ( D and V DS for FET) Why controlling the biasing? S-parameters or similar are only valid at a specific operating point (small signal parameters) temperature essentially controls the properties of the transistor. More so on BJTs than on FETs

5 Voltage or Current Drive of the Transistor Voltage drive: The collector current increase exponentially wrt the base voltage VBE V BE Current drive: The collector current depends linearly wrt the base current

6 Voltage Drive of Bipolar Transistors Simplified relationship: S V BE V e T VBE 1 e V T S VBE where the thermal voltage V T kt q 26 mv T 300K k = Boltzmann s constant = [J/K] T = absolute temperature [K] q = charge of the electron = [C] The saturation current S varies between different transistors V T and S are both temperature dependent

7 Temperature Dependence at Voltage Driven Biasing S A A 80mA T C 55 A T C Example: Calculate the collector current at constant V BE V BE 0, 7745 V V BE V BE V S e T 1mAatT 300 K -20 to 80 is a standard temp. range for a commercial design

8 Temperature Dependence at Current Driven Biasing 0 0 T C The temperature dependence at current driven biasing is considerably friendlier compared to voltage driven biasing! A factor of 2 compared to 2000!

9 Controlling the Operating Point The operating point needs to be controlled to avoid thermal avalanche effect that might destroy the device Three common methods to implement the feedback: VCC VCC passive current or voltage driven biasing R B R C C R C1 R 1 R 2 T 2 C2 C1 T 1 active biasing V CC R C2 thermal feedback T 1 D V D C1

10 Current Driven Biasing The simplest circuitry: Moderate temperature dependency Sensitive to variations in the current gain, 0 Requires large resistance values Loop gain: 0 R C R B Large loop gain if the ratio V CC / V CE is large R B V CC R C C Alternative circuit solution: Not strictly current driven Moderate temperature dependency Less sensitive to variations in the current gain Does not require large resistance values Loop gain : 0 R C R B2 R B3 R B1 V CC R C C R B3 R B1 R B2 Large loop gain if the ratio V CC / V CE is large R B2

11 Voltage Driven Biasing Series feedback Decent temperature dependence Not sensitive to variations in the current gain R C may be replaced with an RFC * Loop gain: g m R E Not suitable at high frequencies due to difficulties to signal ground the emitter without introducing stability problems R B1 R B2 V CC R C C R E *RFC = Radio-Frequency Choke, a large reactance coil intended for high frequencies

12 Example of Active Biasing Circuit V CC ma without feedback R C1 R 1 R 2 T 2 C2 R C2 C1 T 1 active biasing passive current drive T C T 2 : low frequency transistor working as current generator T 1 : high frequency transistor

13 Control by Thermal Feedback V CC Thermally connected D C1 T 1 V D The diode V D is thermally connected to the transistor Good in PAs etc. where high currents heat it up

14 Biasing of Field Effect Transistors The FET shows a slight temperature dependence compared to the BJT Large spreading in the threshold voltage compared to the BJT Only voltage driven biasing possible Passive biasing circuits are not usable if there is a large variation in the threshold voltage

15 solating the Bias Design from the Signal Design Example using RFC s biasing circuit signal output signal input signal circuit RFC = Radio-Frequency Choke may be used up to medium high frequencies

16 solating the Bias Design from the Signal Design Example using stubs biasing circuit signal input signal circuit signal output At high frequencies the RFC s are replaced with line elements

17 Lab 3 RF amplifier