Microwave Amplifier Biasing Made Easy

Optimal amplifier biasing can make a direct impact on the performance of your system. However, choosing the correct bias levels for your application can be challenging, and there are a number of factors that are often overlooked. In this short (25 minutes) webinar, we will provide insight into:
• Important distinctions between amplifier topologies
• Biasing considerations to improve noise filtering, reliability, and electrical performance
• Heat: why it’s a stronger factor than you might think

Q & A

How much sooner than the positive bias does the negative need to be applied?

For depletion mode FETs and HEMTs, we mentioned that the negative gate bias should be applied before the positive drain supply voltage. We believe that it would be roughly on the order of nS-µS that the gate would need to be applied prior to the drain voltage. Through simulation, we’ve discovered that the gate seems to turn on much faster than the drain voltage.  Because of this, I believe that it may be more dangerous turning voltages OFF together than turning them ON together. In our lab experience, we have not noticed any damage occurring when applying the gate and drain voltages at the same time, but this may result in reduced long-term lifetime.  In many circuits, you will see a momentary ringing effect in the actual applied voltage for extremely rapid simultaneous turn-on or turn-off, but in most application circuits there is enough capacitance in the power supply and/or the bias lines to prevent any serious danger from this.

If you have bias oscillations in your system or test, how do you track them down? How do you fix them?

Oscillations can be difficult to discover when using just a network analyzer to test your amplifier. The best way to track them down is to use a spectrum analyzer. First, apply the DC supply and bias voltages and monitor the output of the amplifier for any unknown spurs while sweeping the spectrum analyzer. If you see any spurs, then you know you have an oscillation at that frequency. Next, you can input a single tone frequency using a signal generator and sweep that frequency to check for any unwanted spurs that arise. It is also important to recognize that oscillations could occur at any bias point or frequency combination, so you have to move around the frequency while monitoring the output in kHz-MHz ranges around the signal frequency. It’s also helpful to decrease the IF bandwidth on the spectrum analyzer and zoom in around your tone frequency to get higher resolution. Oscillations often occur at low frequencies and then mix up with the fundamental frequency, creating a periodic arrangement of spurs that resemble a picket fence on your spectrum analyzer monitor.

To fix these oscillations it usually requires a lot of trial and error. It typically requires adjusting the bias circuitry with different bypass arrangements. Multistage amplifiers can have unwanted feedback paths that create a in-phase positive gain loop that will cause output power suck outs, other spur mixing tones, and increased power consumption. This can often be a subtle problem that should be tested if you are having issues in your system.  If you have any issues of this kind with any Marki amplifiers, we encourage you to reach out to [email protected] for assistance.  We have spent a lot of time trying to protect against these issues, and will happily share what we have learned about our parts.

Does it make a big difference if I bias for a fixed current, like most pHEMT amplfiiers say I should, or just use a fixed bias?

It typically does not make a big difference. However, if you are trying bias for very repeatable performance, then you should bias for a fixed current. We have noticed that for pHEMT amplifiers we can see as much as 5-10% variation in the current pull with a particular gate bias, so it’s best to design for a fixed current if you are after repeatability. However, this can be a pain for most of our customers to design around a fixed current pull. Therefore, on our datasheets we demonstrate performance for fixed gate and drain biases.

Will you provide specific harmonic data for your amplifiers on request?

Yes! We are happy to take specific measurements for our potential customers.

how do the various packages (bare die, SMT, and module) affect performance and how do you mitigate that in your design?

Bare die is the cleanest version of the amplifier because you don’t have to account for the extra parasitics you get from the packages. Surface mounts have the most extra parasitics from the vias which add some capacitance and inductance. The wire bonds from the vias that are connected to the board add additional impedances, which is why the surface mount packages have the most variation of the three. Modules typically show similar performance to the bare die, except they might have a minor increase in insertion loss from the extra transmission lines in the package.

What is the max bias current supply for Marki sequencer circuit? Does it have turn off sequence?

The current necessary to operate the charge inverter causes an approximate 30mA increase in the supply current draw. The amplifier itself can potentially run up to 400mA of current pull when driven with high input power, and that is what we have listed as our safe operating maximum on our datasheet for the AMM-6702UC5. If you turn on the supply voltage on the order of µS, then the current to the amp will not turn on before the gate bias is applied. However, if you turn on the supply voltage to the sequencer very quickly, on the order of nS, then you do notice some overlap between the gate and drain bias. The design is safe to the turn off sequence and we have not noticed any problems with how fast you turn off the supply voltage.

For distributed amps, we’ve recently seen some trends in bias that differ from typical Class A amplifiers. In particular, we’ve seen second order product performance correlate more strongly to drain current, and third order product performance correlate more strongly to drain voltage. Do you have any insight on what causes these trends and whether they may be fundamental properties of these amps or perhaps specific to individual designs?

The typical slope of a class A amplifier load-line is for a particular impedance presented to the transistor at a particular frequency, and distributed amplifiers are typically designed for broadband applications. Therefore, distributed amps are only truly class A at one frequency, and as you go across the band the slope of the load line will move around quite a bit, which will impact the linearity and the appearance of harmonic products.  If you change the bias voltage, you’ll change the frequency at which you have your best power match, and you’ll overall increase the linearity of the amplifier a bit by creating more voltage headroom.

Regarding the third order products, typically for a stack/cascode of amplifiers you will see the third order products increase as the amplifier is driven with higher input powers, regardless if it is a distributed amplifier or not. This means that if you have more available output power before you enter saturation, then the third harmonic generation will not be as present at lower input powers. As you increase the drain current, you will see lower power third order products and correspondingly higher third order intercept.

Do any of the HBT amps need sequencing, or is it just the pHEMT amplifiers?

Just the depletion pHEMT amplifiers require sequencing. The HBT’s base voltage is controlled by a current mirror that does not allow much current to flow through the collector of the RF transistor unless there is sufficient positive bias voltage applied. Only the AMM series of amplifiers require sequencing in their turn on procedure in Marki’s catalog, and that does not include the new UC5 package which is internally sequenced.