Q. What is the effect of using a 50Ω amplifier in a 75Ω system?
A. When a 75Ω load is seen from an ideal 50Ω amplifier or vice-versa, a 1.5:1 VSWR results which alters gain, output return loss, and gain flatness in real-life amplifiers. If active directivity (defined as isolation minus gain) is low, a change in load impedance will result in a change of input impedance and a change in source impedance will result in a change in output impedance. Hence, a 75Ω load on an amplifier with low directivity will affect the input impedance of an amplifier. Maximum transfer of power may not occur. However, in many applications, the mismatch may not be objectionable. For specific performance details, the 50Ω amplifier should be tested under 75Ω conditions.
Q. What is output VSWR and what is its significance?
A. Output VSWR is a measure of how much power is reflected back from the amplifier’s output port when an external signal is applied to that port. VSWR varies from a theoretical value of 1:1 for a perfect match to greater than 20:1 for total mismatch. Since loads in practical applications vary with frequency, maximum power and gain flatness will also deviate from what is specified. If the amplifier is connected to its load by a cable, and all three have different impedance. Multiple reflections between the amplifier and its load can occur resulting in greater variation in frequency response. In general, the output impedance (characterized by output VSWR) is the source impedance of the following device.
Q. How is output VSWR measured?
A. A simple setup using a directional coupler is shown below:
First, establish the 0 dB reference as follows. Apply the input signal to the directional coupler output port as shown. Apply a short circuit to the coupler’s input port and measure the power at the coupled port. Then replace the short with an open circuit and note the reading at the coupled port. The average of the two readings is the 0 dB reference. Next, substitute the open circuit with a 50Ω load. Note the reading; this will give you the measurement range of the setup. Remove the 50Ω load and replace it with the DUT. Measure how far the reflected signal is from the 0 dB reference; this is the output return loss (RL / return loss). To convert output return loss to VSWR, use the formula. For more accurate measurements, use a vector network analyzer.
Q. What is the relationship between reflection coefficient, VSWR, and output return loss?
A. The voltage reflection coefficient (ρ) is the ratio of the reflected to incident voltage in an amplifier or device. The theoretical reflection coefficient varies from zero for a perfect match to one for a total mismatch. Magnitude of reflection coefficient and VSWR are related by:
Return loss is related to the magnitude of reflection coefficient |ρ| by:
Q. To improve matching, can I use a resistive pad between amplifier stages?
A. Of course. But at the expense of overall gain, noise figure, and/or output power. The higher the gain of the first stage and the lower the value of the attenuator, the less the degradation of noise figure. Overall noise factor is calculated as follows:
where F1, F2 are noise factors of first, and second amplifiers, and L is the loss factor of the pad.
Noise figure in dB = 10 log10F, where F is noise factor.
Loss in dB = 10 log10L, where L is loss factor.
Gain in dB = 10 log10G, where G is gain factor.
Q. What is the significance of an amplifier’s directivity characteristic in a system design?
A. Directivity is the difference between isolation and gain. Directivity is an indication of how the impedance mismatch at the amplifier’s output affects the input.
In receiver applications, a filter following a wide-band amplifier rejects high-frequency spectral components generated by the mixer, by reflecting them back to the amplifier. If the amplifier has poor directivity, the reflected components will reach the mixer and could affect mixer performance adversely.
If the amplifier provides high directivity, the reflected signals reaching the mixer will be much lower in magnitude and thus cause little interaction at the mixer stage.
Another common application is two-tone, third-order IM testing, where the two-tone signals must be well isolated; amplifiers with high directivity are used between source and combiner.
For relatively high RF frequencies, isolators can be used but they are expensive; for frequencies below 1 GHz, they are difficult to find. High-directivity amplifiers, such as Mini-Circuits’ MAN-AD series, are recommended for such applications.
Q. Can I obtain higher power output by paralleling amplifiers?
A. Yes, but it’s not as simple as merely connecting the two inputs and the two outputs in parallel. It involves judicious use of power hybrids with proper amplitude and phase balance and power levels, as well as amplifiers well matched for gain and phase characteristics. Examples are the HELA-10 and MERA series of amplifiers.
Q. I want to vary an amplifier’s gain. Can I adjust the amplifier’s supply voltage to achieve an AGC effect?
A. It’s not recommended. An amplifier is designed to operate at a given supply voltage and its performance specifications (gain, power output, saturation, frequency response, etc.) are based on the stated supply voltage. Boost the supply voltage too much and gamble on the amplifier burning up; reduce the supply voltage too low and expect the performance specs to deviate considerably.
A voltage-variable attenuator such as ZX73-2500 can be used with the amplifier to vary the overall gain.
Q. I’m working with a 12V system and am considering your +30 dBm ZHL series amplifiers such as ZHL-42. The specs indicate a +15V supply is required. How will performance be affected with 12V rather than 15V DC input?
A. Typical performance data are available on the Mini-Circuits website for many amplifier models at three DC power supply voltages, such as +12, +15, and +16V. https://www.minicircuits.com/products/RF_Amplifiers.html .
Q. My application involves injecting sharp RF pulses to an amplifier. How can I tell whether the amplifier’s peak power limits will be exceeded?
A. Here’s a conservative rule-of-thumb estimate for a 50Ω amplifier:
- Take the amplifier’s maximum input power rating.
- Convert from dBm to W.
- Multiply by 100.
- Take the square root, and use this figure as the maximum peak signal in V that can be applied.
Q. When an amplifier is used in a test setup, is there such a thing as a safe sequence to connect the amplifier’s input, output, and supply voltage to avoid damage?
A. Yes, there is a recommended procedure. Begin by connecting the load, then the DC supply, and finally, the RF input signal. When finished, first disconnect the RF input, then the DC power, and finally the load.
Q. Please sketch the test setup and describe the test procedure used to measure an amplifier’s second-order intercept point.
A. A block diagram of a set up for measuring amplifier two-tone distortion is shown below. The product F1 + F2 of an amplifier is quantified and specified in terms of its second order intercept point (IP2). In the linear region of the amplifier, if second harmonic is A2 dB below fundamental, this IP2 is given by:
IP2 (dBm) = Pout (dBm) + A2
In the block diagram, a low-pass filter is provided to attenuate second harmonics of the generator 10 to 20 dB below that generated by the device under test (DUT). Sufficient attenuation should be provided at the DUT output to prevent spectrum analyzer from generating harmonics.
Q. Describe the procedure for measuring 2-tone 3-order intercept point of an amplifier.
A. A block diagram of a setup for measuring two-tone third-order intercept point (IP3) is shown above.
If F1 & F2 are the frequencies of the two tones, then 2F1 – F2 and 2F2 – F1 are the third-order products. The setup should ensure that second harmonic of F1 & F2 are at least 10 to 20 dB below the third-order products to be measured. Care also should be taken to prevent F1 & F2 interaction and generation of third-order products in the instrumentation. Amplifiers 1 and 2 are selected such that they have high directivity. This provides the desired isolation of the generators. If POUT is the desired signal, and A3 is the level of the third-order product below the desired signal, then the output third-order intercept point is given as:
IP3 (dBm) = POUT (dBm) + A3 ÷ 2
Example: Let Pout = +10 dBm, and let the 2-tone third-order products each be – 30 dBm (40 dB below POUT). Then, IP3 = +10 + 40 ÷ 2 = +30 dBm.
Q. Does the Gain spec of a MMIC amplifier such as ERA-2SM+ have to be adjusted by 6 dB to account for the 50Ω source and 50Ω load impedances?
A. No, “gain” is actually insertion gain, which is the dB-increase in output power observed when the amplifier is inserted between the 50Ω source and 50Ω load.
Q. Please explain the specifications in amplifier data sheets headed “Maximum Power Output”.
A. The quantity under the sub-heading “(1 dB Compr.) Min.” is the minimum value of output power at 1 dB gain-compression (P1dB).
Most amplifiers start to compress approximately 5 to 10 dB below P1dB. Applying signal power levels above this point results in a decrease in gain; therefore, the change in output power will not be linear with respect to a corresponding change in input power to the point where the amplifier is at saturation (PSAT) and the gain equals zero. Operating at output levels above P1dB is not a normal operation for a linear amplifier.
The quantity under the sub-heading “Input (no damage)” is the value that must not be exceeded at any time; if that power is actually applied, the amplifier will be in saturation.
Q. What is the maximum operating value of junction temperature for MMIC Darlington amplifiers, such as ERA or GVA series?
A. The maximum operating value can be found as follows:
- From the data sheet of the desired model, calculate power dissipation as the product of recommended device operating current (Max. value of device operating current for GVA) and max. value of device operating voltage.
- Calculate junction-to-case temperature rise as the product of that power and “thermal resistance, junction-to-case”.
- Calculate the junction temperature as the sum of (a) that temperature rise, (b) 85 Ω, and (c) the temperature-rise-above-ambient value given in the note next to the Absolute Maximum Ratings table.
Example, for GVA-84+: 0.13A x 5.2V x 64C/W + 85 + 10 (typical temperature rise due to the thermal resistance of the user’s PC board to ambient) = 138℃.
Q. Can the monolithic Darlington amplifiers such as the ERA, Gali, GVA, MAR and MERA series be operated at low RF frequencies down to DC?
A. These MMIC Darlington amplifiers can be used at as low a frequency as desired, by user’s choice of blocking capacitors. However, they cannot be DC-coupled. The reason is that they generate internally a DC bias voltage at the input terminal, which would be upset by using DC coupling.
Q. MMIC amplifier data sheets give a typical value for thermal resistance junction-to-case. How can I determine thermal resistance junction-to-air, in still-air condition?
A. The process is given below, using Model Gali-74+ as an example. The data sheet on the Mini-Circuits website for that model has a typical value for thermal resistance junction-to case: 120 ℃ / W. A note under the “Absolute Maximum Ratings” table in the data sheet states that operating temperature is based on typical case temperature rise six degrees above ambient. That is for PC boards commonly used for this type of component, in still air condition. Given the typical Gali-74+ power dissipation, 0.080 ampere × 4.8V = 0.384W, we can estimate the thermal resistance from the case to ambient as 6℃ / 0.384W = 15.6℃ / W. Therefore, thermal resistance junction-to-ambient is the sum of these values: 120 + 15.6 = 136℃ / W.
Q. For worst-case MMIC amplifier junction-temperature calculation, should the absolute maximum values of operating current and input power be used?
A. No, because the absolute maximum values of current and input power in the data sheet are never to be exceeded and are not intended for continuous service. Use the recommended device operating current and the maximum specified device operating voltage to calculate worst-case power and junction temperature. RF output power, especially when operating at or near the amplifier’s 1 dB gain compression point, can be a significant fraction of the DC power. Because the RF output power is delivered to an external load instead of being dissipated as heat, it can be subtracted from the DC power when calculating the junction temperature. (This effect is known as “RF cooling” of the amplifier.)
Q. Why do some amplifiers such as ZX60-14012L+ have multiple operating temperature ratings: case and ambient?
A. The user can work with either the case temperature rating or the ambient rating for such an amplifier, as they are equivalent. An amplifier dissipates a significant portion of its DC input power as heat, causing the case temperature to rise above ambient air. The higher the DC voltage among the rated values, the higher is the case temperature rise at the same ambient temperature. To keep the case temperature within reliability goals, the maximum ambient rating is reduced at higher DC voltage.
Q. ZRL- series amplifiers have dual temperature ratings: ambient and case. Do these amplifiers need a heat sink?
A. Operating temperature range of ZRL- series amplifiers is rated at -40 to 60℃ ambient, and -40 to 80℃ case. A heat sink is not required if ambient air does not exceed 60℃. Above that temperature, a heat sink or forced-air cooling should be used so that the case temperature does not exceed 80℃.
Q. Some amplifiers such as ZHL-20W-13 are offered with and without the heat sink. If I buy it without the heat sink (such as ZHL-20W-13X), what external heat-sinking do I have to provide?
A. A note on the data sheet tells the user the maximum base-plate temperature for the X-suffix model, and states the thermal resistance required via the user’s external heat sink. Using ZHL-20W-13X as an example, a heat sink with thermal resistance of 0.3℃ / W is needed to limit the base-plate to 85℃ in a 65℃ ambient environment. The temperature rise with such a heat sink is 20℃. Suppose your actual maximum ambient is lower, say 50℃. Then, the 85℃ maximum base-plate temperature still applies, but the 35° rise above your maximum ambient allows you to use a heat sink with worse (greater) thermal resistance: 0.3 X 35/20 = 0.525℃ / W.
Q. If I apply DC power to a high-power amplifier such as ZHL-10W-2G+ by increasing the voltage slowly, I notice that when DC current starts to flow it is initially higher than expected, and then it decreases as I continue to increase the voltage to the normal operating value. Is this a fault?
A. It is normal behavior. This kind of amplifier contains a switching voltage regulator that automatically adjusts the current taken from the user’s power supply to compensate for change in DC voltage. The result is nearly constant DC power consumption and RF performance. For example, DC current drawn by ZHL-10W-2G+ is typically 4.25A at 21V, 3.7A at 24V, and 3.5A at 26V.
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