by Robert Maurer
- AMM-6702UC5 offers positive only, +5V operation of the AMM-6702UC amplifier
- Supply voltage suppression of 100 dB
- The voltage rectifier circuit and BoM are shown below so customers can integrate the design into their own system
- Custom amplifier-level bias voltages are possible by modifying this circuit (contact [email protected] for details).
Marki Microwave recently launched the first products of its AMM amplifier line. The AMM amplifiers are designed using a depletion mode pHEMT process that offers very high bandwidth and efficient mm-wave performance, but unfortunately also requires voltage sequencing and a somewhat uncommon 3V-4V drain voltage which can make it inconvenient for customers looking to design it into a system. Now, customers can avoid the issue of sequencing and negative voltage generation all together using our new UC5 single-supply voltage sequencer.
The Marki UC5 single-supply sequencer board works in conjunction with the AMM amplifier to allow the user to safely apply a single standard 5V positive supply voltage without any sequencing or negative supply voltage. The board internally generates a negative voltage using an inverting charge pump, then feeds it to a sequencer which prevents current from reaching the Vd port of the amplifier until a sufficient negative voltage is applied. Step diodes and a voltage divider are then used to provide a -0.5V negative voltage and a 3.5V positive voltage to the amplifier. The custom UC5 housing features the RF performance of the AMM-6702UC with a single 5V supply pin for the customer to use. We are also providing the circuit diagram of the board in this tech note so our customers can recreate it on their own.
Overall Circuit Diagram
The circuit diagram for the UC5 board can be found below in Figure 1. We designed our version on 63 mil thickness FR4 board, but the customer is free to use whatever material they like:
Figure 1: Full UC5 Board Circuit Diagram
All diodes (D1) on the circuit are SBR0240LP5 (datasheet), which drops 0.5V at 200 mA of current and 0.25V at 1 mA of current. We can break the circuit diagram up into 2 key parts – the sequencer and the charge inverter.
Let’s first look at just the sequencer alone. The circuit diagram is below in Figure 2.
Figure 2: Sequencer Circuit Diagram
What we want from the sequencer for proper operation is for current to be blocked from flowing from the “Applied Voltage” terminal to the “Chip Vdd” terminal until after sufficient negative voltage is applied. We use 2 discrete transistors to accomplish this, the IRLML6401PbF (datasheet) is a P-Channel power MOSFET, and the MMBT2222A (datasheet) is an NPN BJT.
When there is no negative voltage applied, the NPN transistor is off. No current flows through the NPN transistor, which means no current is pulled across R9 and R6 in the diagram above. This means the voltage at both the S and G terminals of the MOSFET are equal to the applied voltage and the VGS of the MOSFET is 0V. When there is no VGS on the P-channel MOSFET, no current can flow through the MOSFET, and therefore, no current can reach the Chip Vdd port.
When a negative voltage of roughly -1.1V is applied, the base-emitter saturation voltage of 0.6V is reached for the NPN transistor. 1 mA of current flows into the collector, dropping 4V and 1.5V across resistors R9 and R6 respectively. This creates a negative VGS well below the threshold voltage of the P-channel MOSFET. Once VGS is more negative than the threshold voltage, current will flow freely through the MOSFET.
Next, let’s look at the Charge inverter circuit. The circuit diagram is below in Figure 3.
Figure 3: Charge Inverter Diagram
The charge inverter circuit uses a Maxim MAX889T (datasheet) high frequency, regulated, 200 mA, inverting charge pump. The charge pump circuit uses a built-in oscillator, power MOSFET switches, and some additional control circuitry to force current through a voltage divider, producing a negative voltage at the “OUT” pin of the MAX889T. We recommend looking through the datasheet, as our inverter circuit is essentially a duplicate of the application circuit suggested in their datasheet with the resistance values tweaked to fit our particular voltage requirements for the AMM process.
Customizing the bias condition
The RF performance of the AMM-6702 using with the UC5 board will be roughly the same as when using the amplifier with 3.5V/-0.5V bias condition. In order to change the bias voltage conditions for higher gain, higher power, higher efficiency operation, the number and type of diodes used in the “step down from 5V to 3.5V” block can be used to adjust the drain voltage and the ratio of resistors R3 and R1 can be adjusted to alter the gate voltage (keep the sum of R1 + R3 at 1.6kΩ to avoid altering the voltage coming from the charge inverter block). If you would like a particular bias condition in a Marki-built AMM-6702UC5 module, contact [email protected].
DC Bias Characteristics
The amplifier level Vg and Vd voltages vs. applied UC5 voltage can be seen below, as well as total current vs. applied voltage. The negative gate voltage generation activates around 2.5V of applied bias and the drain voltage activates around 3.3V of applied bias. As the applied voltage is increased above these voltages, the gate voltage becomes steadily more negative and the drain voltage becomes steadily more positive. This means that in terms of RF performance, the user can expect that the amplifier will have the highest gain around 3.5V of applied bias, but will have higher output power around 5.4V when using the AMM-6702UC5. This circuit is designed to provide close to our nominal 3.5V/-0.5V bias condition at 5V of applied bias. When applying the supply voltage, it is recommended that the user either.
RF S-parameters and saturated output power can be seen below for different applied voltages to the AMM-6702UC5. While gain increases with lower applied voltages, output power and return losses improve with higher applied voltages.
Suppression of the Inverter Oscillator Frequency
While the charge inverter does have an oscillator frequency somewhere between 1.5 MHz and 2.5 MHz (according to the MAX889T datasheet), we have observed that the suppression of this tone compared to the fundamental output of the amplifier is roughly 100 dB. This is very low because the amplitude of the oscillation is proportional to the negative current draw. The amplifier draws virtually no gate current, so it is entirely drawn by the voltage divider. Below is a screenshot from a spectrum analyzer of a 40 GHz tone generated by a frequency synthesizer with 0.73 dBm input tone (Figure 4) and a second screenshot of that input tone amplified by the AMM-6702UC5. There is a tone offset from the fundamental by 260 kHz (marker D3 in the screenshots) which is generated by the frequency synthesizer and then amplified by the amplifier with very little difference in tone suppression. More notably, in this unit, there is a tone at 1.858 MHz (marker D2) offset from the fundamental which we believe is generated by the oscillator in the charge inverter. It can be seen that when the sequenced amplifier is operating, this tone appears above and below the fundamental with roughly 100 dB of suppression.
Figure 4: Input signal generated by frequency synthesizer
Figure 5: Output signal from the AMM-6702UC5 showing 100 dB suppression of inverter oscillator tone
The power consumption of the amplifier + UC5 sequencer board also increases. Including the step-down diodes, the DC voltage increases from 3.5V to 5V, and the current consumption increases by roughly 30 mA from 200 mA to 230 mA. The power consumption therefore increases from 0.7W in small signal to 1.15W in small signal. The current consumption under large signal RF operation will increase with input power as seen in the AMM-6702 datasheet.
UC5 5V Single Supply Voltage Sequencer Board
This tech note outlines the performance of our new AMM-6702UC5 and provided details of how customers can make it themselves with any AMM-6702 amplifier package or any future Marki AMM-line amplifiers to eliminate the hassle of sequencing and dealing with non-standard voltages. Please contact [email protected] to inquire about purchasing options or [email protected] for help with the sequencer design for your own system application.