It appears that the exploration of and the colonization of planet Mars will require a multitude of communication satellite constellations, The present effort is gearing up using application specific cubesats and similar technology. One of the key elements in these small satellites is the Solid State Power Amplifier- the SSPA. Of necessity these amplifiers have to be very efficient and consume very little space. They will be have to be robust and may need multi band capability. Please contact us for more on this important topic. Visit our website for more informative articles and white papers.

RF transformers are not ideal. In order to explore the non ideal RF transformer we need to look at the equivalent circuit of the RF transformer and look at the effects the non- idealities have on it. Interested parties can go to our website and find the brief expose of these non idealities under the “complementary items” menu item.

In general, insertion loss is defined as the loss in signal power, when a device is inserted in the transmission path of a signal on a transmission line or circuit.Insertion loss is usually stated in decibels (dB).

So if the transmitted power is PT and the received power at the load, after the insertion of the device is PR , then the insertion loss is defined as

IL ( dB) = 10Log(PT/PR)

Where the Log is to the base of 10.

Insertion loss is also defined for filters as the ratio of the ouput signal level in a test configuration without the filter installed, to the signal level with the filter installed. So if the output signal level without the filter is V1 and the signal level with the filter installed is V2 then the insertion loss in dB is:

20Log(|V1|/|V2|) dB

If using scattering parameters use the following expression:

10Log(|s21|² /1 – |s11|² )

where the symbols have their usual meaning.

Transfomers also have an insertion loss specification. This is a figure of merit for a RF transformer.

The low end ( or low frequency) loss is determned by the primary inductance. The high frequency insertion loss is dependent on the losses in the inter – winding capacitance and the series inductance.

In addition, for transformers with metallic cores, the permeability is directly proportional to the temperature of operation. As the temperature decreases, the permeability decreases which causes an increase in the insertion loss.

The RF transformer is a very useful device for the design of many types of RF circuits. It can be used as a device for changing voltage and current levels in a circuit and matching impedances.

This very brief post is simply a reminder of the absolute basic design equations for the RF transformer (and indeed, any transformer).

1. n = N2/N1 , where n is the turns ratio of the two winding transformer. N2 and N1 are the number of turns of the secondary and the primary.
2. V2 = nV1, V2 is the secondary output voltage, V1 is the primary voltage.
3. I2 = I1/n. I2 is the current out of the secondary, I1 is the primary current.
4. Z2 = n*n*Z1. Z2 is the impedance seen looking into the secondary and Z1 is the primary impedance. These are very basic quantities of an ideal transformer. For second and third order effects please see the equivalent circuit model of the  transformer. Please visit the SPG website for  more interesting information.

In a system where several amplifiers or other functional blocks are cascaded the calculation of noise figure is :

F = F1 +F1/( G1) + F2/(G1*G2) …..

Here the Fi are the Noise figures of the various stages and Gi are the gains ( or losses) of the various stages. F is the cascaded noise figure.This is, of course, a very basic textbook result but instructive nonetheless. Please visit our website for more technical and other information.

The directional coupler is a very useful device that can be used to sample signals for analysis or control. Typically a directional coupler is used to return part of a RF power amplifier signal back to a control circuit for various functions such as controlling the output power or monitoring the RFPA performance. It is a four port device. Two ports are input port and output port which along with the connection, constitute the main line. The third port is the coupling port where the sampled signal appears. Finally there is the isolation port which is terminated in the characteristic impedance. Ideally no signal should appear there.

The directional coupler is defined by the following specifications:

Some suppliers of directional couplers are : Macom, Mini-circuits, Pulsar, Agilent, Narda Microwave .

Directional couplers can also be designed on – chip and on a board. If you need a custom directional coupler for on chip or on board application please contact us. Please visit the SPG website for more content of interest.

The following is a list of classes of RF Power amplifier and associated efficiencies:

Class A: 25% to 50%,

Class B: 78.5%;

Class AB: 50% to 78.5%.

Class C: Up to 90%.

Class D: Over 90%,

Class E: Over 80% .

Class F: 100% (ideally).

Class G: Depends on input signal power . 60% to 80%.

Class H: 80%

Please visit our website for more information and technical content.

When designing a bipolar transistor the collector current density has to be chosen ( and design to be) less than or equal to: 1.602E-17 NCO where NCO is the doping of the epitaxial layer in atoms/cm**3. If this current density is exceeded, the transistor will not perform well. Please visit our website for more items of interest.

A simple way to estimate the noise of the bipolar transistor is to use the  equivalent noise resistance. This post and its associated paper shows how the equivalent noise resistance is derived. Please visit the Signal Processing Group Inc., website and search under “complementary” items to read or download the paper.

A very useful parameter in noise analysis of the bipolar transistor is the equivalent noise resistance. ( See the post on this quantity in this blog). However, another very important noise measure is the Noise Figure ( NF). The question is: What is the relationship between the two. The answer is very simple. NF = RN/RS. Where RN is the equivalent noise resistance and RS is the input signal source resistance. Please visit our website for more technical information and useful articles.