# RF-Amp

## RF Amplifier Features

• Class A (Common Emitter) Amplifier
• Emitter resistor bypassed with capacitor for higher AC gain
• Transformer coupled input/output for impedance matching
• Capacitively coupled input
• Single 2N3904 NPN transistor
• Applications
• IF Amplifier
• Antenna Amplifier
• Microphone amplifier (with minor modifications)
• As RF Amplifier
• Measured Gain Bandwidth (GBW) of 150
• Measured Gain @7 MHz, +25.2 dB at 12V
• Measured Gain @9 MHz, +24.7 dB at 12V
• Measured Gain @30 MHz, +13.8 dB at 12V
• Input connectors: SMA, BNC, or direct solder coax
• +12V nominal power
• 12-14 VDC
• 5 mm terminal block for power
• 49x49mm card
• 4x 4-40 mounting holes

## LT Spice Simulation

### As Built - Rev 1

• Insert 4.7 ohm resistor to emitter bypass capacitor
• Reduces maximum gain
• Increases gain over 1-30 MHz bandwidth
• Possible better for Antenna Amplifier application at lower frequencies
• +22.4 dB at 9 MHz

## Charlie Morris Design Calculations

### Beta DC

• Geometric mean min/max beta at operating current
• =sqrt(100*300) = 173

### Beta AC

• Gain bandwidth product divided by operating frequency
• Assume operating frequency of 9 MHz (IF frequency)
• = 300/9 = 33.3

### DC Operating Point

• Max HFE RF gain at CE current of 10 mA
• If Vce = 6V, this is 60 mW power dissipation
• Assume Ve (voltage across emitter resistor) = 1/10 Vcc = 12V/10 = 1.2V
• R3 is Re (emitter resistor) = 1.2V/0.01A = 120 ohms
• VCE = 0.7V (typical from data sheet)
• V(emitter) at 10% of Vcc rule of thumb = 1.2V
• V(base) = V(emitter) + VCE = 1.9V
• Base current is collector current divided by Beta DC
• Biasing resistors = 10x current needed by base current
• 10 mA in C-E, beta DC less = 10 mA/173 = 58 uA
• 10x the current in the biasing resistors = 580 uA (calculated)
• R2 is 1.9V at 580 uA = 3.29K use 3.3K
• R1 sources current to R2 and transistor base
• Voltage = Vcc (12V) - 1.9V = 10.1V
• Current = 577 uA + 58 uA = 635 uA
• R1 = 10.1 / .635 mA = 15.9K, use 15K

#### Measured DC operating point

• Measured with no input
• Vcc = 11.96V
• Current draw = 12 mA
• Quick test for wiring and more or less correct parts
• Expected 11 mA - close enough
• +BUFF = 11.84V
• 0.12V which is 12 mA through R4 10 ohms - expected
• V emitter = 1.41V
• 1.41V/12 Ohms = 11.75 mA close to 12 mA total measured current
• V on input divider = 2.06V
• Vbase + 0.7V - close
• Measured Vbe = 2.06-1.41 = 0.65 - close

### Input resistance

• Xc for 0.1uF cap from emitter to ground
• C=0.1uF
• F=10MHz
• 1/2*pi*F*C = 0.16 ohms
• Parallel resistors R1, R2 paralleled with transistor input impedance
• R1=15K, R2=3.3K
• Transistor resistance = Beta AC (33.3) times re
• re = 26 / Ie (10 mA in mA) = 26/10 = 2.6
• SSDRA uses 25 as constant - close enough
• 26 comes from Ebers-Moll approximation
• Beta AC * re = 33.3*2.6 = 83.3 ohms - predominates
• All in parallel are 80.8 ohms

### Gain calculation

• Approximation
• Ic = 0.01A
• Rc = 200
• Vrc = 2V
• Gain = Vrc / vt
• vt = 26 mV at room temperature
• Gain = 2V / .026V = 79.2 V/V
• Gain = +37 dB

### Input/Output Transformers

#### Input Transformer

• Input Transformer (T1 on Charlie's - T2 on this board)
• Need to calculate turns ratio
• 50:80.8 Ohms
• n = sqrt(Zout/Zin)sqrt(80.8/50) = 1.27 turns ratio
• Turns choices
• Minimum number of turns
• Rule of thumb - want Xl (coil impedance smallest value) to be least 4-5X the load
• 5 * 80.8 ohms = 404.2 ohms minimum
• More turns = larger capacitance and drops bandwidth
• Toroid is FT37-43
• From Toroid page
• Xl = 404.4 at 9 MHz is 4.5 turns, round up to 5
• Try nearest integer numbers turns ratios
• 5:6 = 6% error
• 6:8 = -4.6%
• 7:9 = -1.1% << good choice
• 8:10 = +1.7%
• 9:11 = +4.0%
• 10:13 = -2.19%
• Use 7:9 turns ratio for optimal input transformer

#### Output Transformer

• Output transformer (T2 on Charlie's - T1 on this board)
• T2 - different than Charlie's design since my Crystal filters are all 50 ohms in/out
• SSDRA suggest presenting 200 ohm load to the collector
• Can't find reference in SSDRA
• Reflecting back 50 ohms load to 200 ohm collector...
• 200:50 ohms
• n = sqrt(200/50) = 2.0:1 turns ratio
• 10:5 turns
• 10 turns primary (on transistor collector)
• 10 turns = 35 uH
• 5 turns secondary (towards output)
• 5 turns = 8.75 uH
• 15 turns = 9.5 in

## NanoVNA Measurements

• Goal: Measure RF-Amp performance using a NanoVNA running NanoSaver software on PC
• S21 (gain) needs to be measured with a 40 dB attenuator on input to RF-Amp to avoid compression on the output
• S11 (reflection) input impedance can't be measured with input 40 dB attenuator because S11 just ends up measuring the attenuator
• Output should be terminated to 50 ohms for S11 measurement
• DC current = 12 mA

### Measure S21

• Put 40 dB attenuator on RF-Amp input, measure S21 at output
• NanoVNA provides 50 ohm load to RF-Amp to properly terminate output
• Measure S21 with 9:11 input transformer
• S21 @ 100 KHz = -8 dB dB
• S21 @ 1.45 MHz = 35.4 dB (peak gain)
• S21 @ 9.1 MHz = 24.3 dB
• S21 @ 16 MHz = 20.1 dB
• S21 @ 30 MHz = 12.7 dB
• Peak gain justifies use of 40 dB attenuator to protect NanoVNA

#### LTspice vs NanoVNA

• LTspice simulation was pretty similar to NanoVNA results
• -10 dB at 100 KHz
• +32 dB at peak
• Lower output at higher frequencies

### Measure Input Compression

• Is there compression if the NanoVNA drives the input directly?
• Test by driving directly from NanoVNA set to CW = 9 MHz
• Measured output with scope - not clipped at 9 MHz
• Approx. 1Vpp input = +22.1 dBm gain which matches the S21 with the attenuator on the input
• Vpp = 12.4V with 50 Ohm load resistor
• Starts clipping at 7 Mhz and down
• Therefore, can measure input impedance at 9 MHz
• Other evidence of compression
• Compare S21 gain with no input attenuator, put external 40 dB RF Attenuators on output of RF-Amp to protect NanoVNA input
• S21 shows lower gain in lower frequencies so clipping/compression is happening
• Was: 35 dB at 1.4 MHz
• Is: 23.1 dB at 1.5 MHz
• Due to compression can't accurately measure lower frequencies with attenuator at output
• Compression below 7 MHz matches what was on scope

## W2AEW S11 Measurement Method

• Can't drive the RF Amp directly from the NanoVNA
• High output level from the NanoVNA overdrives the RF Amp
• W2AEW provides a way of driving the RF Amp card without overdriving and still measure S11

• Calibrate NanoVNA using External 30 dB Attenuator
• Scan 1-30 Mhz
• Overdriven at 1 MHz which "swamps" the RF Amp
• Re-calibrated at 1.5-31.5 MHz
• Peak gain at 1 MHz = 32 dB
• Does not overdrive the Amp or NanoVNA
• Downsize is a lot of noise in the return loss
• Tested two units
• Unit 1 has a 7:10 input transformer (T2) ratio
• Unit 2 has a 7:9 input transformer (T2) ratio

### Unit 1

• 9 MHz measurements
• VSWR = 1.172
• S11 (Return Loss) = -22.014 dB
• S21 (Gain) = +23.624 dB

### Unit 2

• 9 MHz measurement
• VSWR = 1.182
• S11 (Return Loss) = -21.565 dB
• S21 (Gain) = +24.656 dB
• 20 dB gain at 15 MHz
• Gain Bandwidth (GBW) = ~150
• GBW is a good predictor of gain at particular frequencies
• Calculated Gain of 14 dB at 30 MHz - measured at +12.8 dB
• Measured at +26 dB at 7 MHz

## Modified to use as Microphone Amp

• Charlie's video

### Test with Electret Microphone

• Charlie assume voltage/current - didn't measure
• Chose to determine Electret operating point through measurement
• DC powered
• AC coupled output
• 13.8VDC (max) power
• Adjust resistance to get 4V across mic at 13.8 VDC supply
• Selected value = 33K pullup to 13.8V gets 4V across mic
• 2.5V out with 12V supply

### Wiring up Mic to Amp

• Install Electret Condenser Microphone on small perf board
• Cable using 18" RG-174 coax to input of RF Amp card

### Part Value Changes

• No transformers
• transformers replaced by passives/jumpers
• R1 - 15K = OK
• R2 - 3K (small difference vs 3.3K on RF Amp)
• R3 - 120 = OK
• R5 - 50 ohms
• Install R5 to simulate balanced modulator 50 Ohm load
• Install R5 on long leads to easily remove
• T1 primary winding - 560 ohm
• C1 - 0.1 uF
• C2 - 10 uF
• C3 - 47 uF
• Add capacitor from Vc point (transistor collector and 560 ohm resistor) to T1 output side
• Install output SMA connector

### Tested

• In application output goes to Balanced Modulator
• Output level should be +7dBm for ADE-1 Mixers
• Tested into AudioAmp386 - works

### Mic Amp LTspice Simulation

• Low frequency response can be improved by increasing the value of the emitter bypass capacitor