driver amplifier for 7–tesla mri smart power amplifier
TRANSCRIPT
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Driver Amplifier for 7–Tesla MRI Smart Power Amplifier
presented by Kevin Kolpatzeck
supervised by Prof. Dr.-Ing. Klaus Solbach
Institute of Microwave and RF Technology
University of Duisburg Essen
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Introduction
Concept
Power Amplifier
Gate Bias Circuit
Voltage Regulator
Power Monitor
Assembly
Conclusion
Contents
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 2
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7 – Tesla MRI system employs 32 channels, each of which can produce 1 kW of pulsed power at 300 MHz
each channel consists of an RF power amplifier and a cartesian feedback loop
Introduction
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 3
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1 kW – power amplifier:
Driver Amplifier will be used to evaluate some concepts for the driver and high-power stage, e.g.
Temperature Compensation
Power Monitor
Introduction
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 4
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Driver Amplifier can be divided into five sub – circuits:
Concept
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 5
- Operating Point Stabilization
- Class A / Class AB Switching
- Temperature Compensation
- On / Off Switching
- Drain – Source Voltage Stabilization
- Load Regulation
- Easy & precise monitoring of
- pulse power
- momentary power
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Power Amplifier Power Amplif ier Concept
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 6
Power amplifier is built around a Freescale MRF6V2010N
RF power MOSFET
n – channel
enhancement mode
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Power Amplifier Input Circuit
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 7
Input circuit provides d.c. – feed and matching of the 50 Ω – source to the input of the MOSFET
d.c. – feed and first stage of matching d.c. – block
second stage of matching
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Power Amplifier Input Reflection Coefficient S11
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 8
Best match has been achieved with the two–stage matching network, that is however quite different from the manufacturer’s recommendation
Input reflection coefficient S11 has been measured with a vector network analyzer (VNA) for a Class A and a Class AB operating point
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Power Amplifier Input Reflection Coefficient S11
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 9
Small signal (Pin = 0 dBm) input reflection coefficient S11 as measured with VNA
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Power Amplifier Output Circuit
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 10
Output circuit provides d.c. – feed and matching of the output of the MOSFET to the 50 Ω – load
d.c. – feed and first stage of
matching
d.c. – block
second stage of matching
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Power Amplifier Power Gain
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Power gain (forward transmission coefficient S21) is determined by
operating point
input power
quality of the input and output matching networks
Power gain has been measured with
vector network analyzer (VNA)
RF signal generator and power meter
(RF signal generator and spectrum analyzer)
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Power Amplifier Power Gain
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Small signal (Pin = 0 dBm) power gain S21 as measured with VNA
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Power Amplifier Power Gain
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Power gain and P1dB as measured with RF signal generator and power meter
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Power Amplifier Intermodulation Distortion
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Third – order intermodulation products are not affected by output filter, so they can be used to quantify the harmonic distortion caused by the MOSFET.
Third – order intercept point has been measured with a two – tone measurement:
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Power Amplifier Intermodulation Distortion
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 15
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Power Amplifier Summarized Data
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 16
VNA Power Meter Spectrum Analyzer
Datasheet
Input Reflection Coefficient S11 (Pin = 1 mW / 0 dBm)
– 33 dB
Small signal power gain (Pin = 1 mW / 0 dBm)
27.5 dB 27.1 dB ca. 24 dB
P1dB 15.2 dBm ca. 17.3 dBm
Maximum Output Power (Pin = 100 mW / 20 dBm)
19.1 W / 42.8 dBm
12.6 W / 41 dBm
IIP3 23.2 dBm
OIP3 49.5 dBm
These values need some discussion!
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Power Amplifier Discussion
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 17
Small signal power gain and maximum output power strongly exceed the values to be expected from the datasheet.
Explanations:
Measurements have been performed with three different principles, leading to similar results → plausible
Measurements at VGS = 3.4 V (Class A); datasheet values at IDQ = 30 mA (corresponding to VGS ≈ 2.6 V; close to Class B)
Measured input return loss S11 is approx. –33 dB; datasheet values are between –14 and –9 dB
Input and output matching networks are quite different from the networks in the datasheet, both in architecture and component values
→ realized matching networks are supposedly better than the manufacturer‘s
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Gate Bias Circuit Concept
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 18
Operating Point Stabilization
Choosing a Class A and a Class AB operating point
Switching between these two points
Temperature stabilization of the quiescent current
Fast switching between the designated gate bias voltage and VGS = 0 V
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Gate Bias Circuit Operating Point Selection
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Input voltage is stabilized against ripple and noise on the power supply rail with low-pass filter and Zener diode.
One potentiometer (R3) allows the selection of the Class A operating point.
A portion of a second potentiometer (R5) is bypassed by a logic-level MOSFET T1 when a voltage of 5 V is applied at its gate.
This potentiometer determines the distance between the Class A and the Class AB operating point.
TTL input
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Gate Bias Circuit Operating Point Selection
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Only certain pairs of operating points can be reached with this circuit
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Gate Bias Circuit Temperature Compensation
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 21
The threshold voltage of a MOSFET decreases with increasing temperature → the quiescent drain current increases.
The operating point shifts as the device heats up.
dVthdT
= 1 + γ
2 Vinv∙
dVinvdT
with dVinv
dT = − 2.3 … − 1.7
mVK
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Gate Bias Circuit Temperature Compensation
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 22
The forward voltage VD of a silicon pn‒diode follows a similar law as the
threshold voltage of a MOSFET: dVDdT
≈ − 1.7 mVK
Compensation principle:
one pn‒diode is placed far away from the MOSFET (approximately ambient temperature)
one pn‒diode is placed close to the MOSFET (approximately junction temperature)
The difference between the forward voltages is amplified and added to the selected gate bias voltage by a differential amplifier with common mode offset input of type AD8137
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Gate Bias Circuit Temperature Compensation
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 23
Sensor diode is placed next to the MOSFET, not on top because diode picks up electromagnetic field radiated by the MOSFET
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Gate Bias Circuit Temperature Compensation
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V′ Bias = VBias + R5R3
∙ (VD1 − VD2) Reference diode Temperature sensing diode
Common-mode offset
Differential amplifier with feedback potentiometers
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Gate Bias Circuit Temperature Compensation
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 25
The feedback potentiometers around the differential amplifier must be set to the same value for the common mode offset to work properly
A procedure for adjusting the feedback resistors for optimum temperature compensation has been developed.
The quiescent drain current is held constant to within about 10 mA.
Problem: In an enclosure the reference diode will eventually heat up, too.
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Gate Bias Circuit On‒Off Switching
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Pulsed operation: amplifier does not need to be in its operating point when no pulse is present at the RF input
Power dissipated in the MOSFET can be reduced by switching VGS to 0 V between the pulses.
TTL input
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Voltage Regulator Concept
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 27
Power amplifier needs stable drain‒source voltage, especially during the RF pulses
Little current flowing into the drain between the pulses
Significant current flowing into the drain during the pulses
Large transients
Power supply regulator is under stress
Power line inductance hinders transients
Power supply might not be able to provide the required drain current
Solution: charging capacitors and discrete voltage regulator close to the amplifier
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Voltage Regulator Circuit
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Charging capacitor
Voltage regulator
Voltage stabilization capacitor
RF block
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Voltage Regulator Static Line Regulation
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Static line regulation: dependence of the output of a regulator on the input in the static case
slope 1
slope 0.4, i.e. line regulation ca. 4 dB
Regulation threshold
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Voltage Regulator Load Regulation
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Load regulation: dependence of the output of a regulator on the load
ΔV ≈ 66 mV, i.e. load regulation ca. 28.7 dB
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Power Monitor Concept
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Necessity to measure the output power of the amplifier during operation
Difficult because of high frequency and large dynamic range
Solution: AD8307 logarithmic amplifier
converts RF voltage to d.c. voltage (envelope)
outputs a voltage that is proportional to the logarithm of the input voltage
Theoretically output power can be measured with a multimeter this way
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Power Monitor Concept
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 32
Output voltage of the driver amplifier needs to be reduced to within the input range of the AD8307 with a probe circuit
Probe resistor with frequency compensation
a.c. coupling
50 Ω termination
Input equivalent circuit of the AD8307
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Power Monitor Frequency Response
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Low frequency roll-off
suppresses LF hum and
d.c. offset
High frequency roll-off attenuates HF noise and harmonics
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Power Monitor Pulsed Operation
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 34
Gate ON:
RF in:
RF out:
AD8307 out:
Logarithm of pulse output power
Logarithm of noise power
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Power Monitor Sample‒and‒Hold Circuit
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Consequence for measurement of output power:
Analog multimeter: approx. measures average power
Oscilloscope: measures pulse and noise power
Digital multimeter: does not allow any sensible measurement
Solution: Sample‒and‒hold circuit based on an LF398 that holds the output voltage of the AD8307 between the pulses
S/H circuit can be controlled by “Gate ON” voltage because it is supposed to be high only during the RF pulses
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Power Monitor Complete Circuit
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Probe Logarithmic amplifier IC
S/H Circuit
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Power Monitor Calibration
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Relationship between d.c. output voltage and output power must be established
Calibration against a reference power meter
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Power Monitor Calibration
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Linear law between d.c. output voltage and power in dBm:
PPulse = 39.73 VPulse
V ‒ 54.79 dBm
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Assembly Circuit Board Layout
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Assembly Power MOSFET Mounting
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Good source‒ground connection (low resistance, low inductance) and low thermal resistance is important
Glue‒on mounting on a self‒built brass heatsink
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Assembly Power MOSFET Mounting
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Assembly Housing
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Assembly Additional Circuitry
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Signaling LEDs
Inverter for the Gate ON voltage
Polarity protection
Placed on a perfboard underneath the lid of the box
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Conclusion
Kevin Kolpatzeck – Driver Amplifier for 7–Tesla MRI Smart Power Amplifier 44
Complete, conveniently usable amplifier for amplification from 100 mW to 10 W
Amplification and maximum output power higher than expected
Relatively poor distortion figures
Concepts for use in driver and high power amplifier have been tested and proven functional and useful
Operating point flexibility
Temperature compensation
Gate shutdown
Power Monitor
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Thank you for your interest!