Abstract: Logarithmic amplifiers which have a logarithmic transfer characteristic are used, for example, in microwave instrumentation. Such an amplifier comprises a series of MESFET distributed amplifiers (DA1-DA8), each having a substantially linear transfer characteristic, connected in cascade. An input for the logarithmic amplifier is connected to an input end of the gate transmission line (G) of a first distributed amplifier (DA1) and the output end of the gate transmission line of each distributed amplifier is terminated by a demodulator (DM1-DM8) for demodulating amplitude modulation. Summing means (OP1) are connected to the output of each of the demodulators (DM1-DM8) to sum their outputs and thereby provide the output of the logarithmic amplifier.

Abstract: Logarithmic amplifiers which have a logarithmic transfer characteristic may be used in microwave instrumentation. Such an amplifier includes a series of MESFET distributed amplifiers (DA1-DA8) connected in cascade, with each of the MESFET distributed amplifiers (DA1-DA8) having a gate transmission line (G1-G8) and a drain transmission line (D1-D8) which share a common ground or source line (5), and a number of MESFETs (1-4) connected in parallel with their gate electrodes (g) connected at spaced intervals along the gate transmission line (G1-G8) their drain electrodes (d) connected at spaced intervals along the drain transmission line (D1-D8) and their source electrodes (s) connected to the source line (5). A low pass filter (LPF1-LPF8) is connected to the downstream end of the gate transmission line (G1-G8) of each distributed amplifier (DA1-DA8) with each low pass filter (LPF1-LPF8) passing only the rectified component of the travelling wave passing along each gate transmission line (G1-G 8).

Abstract: A wide band amplifier for operation at very high frequencies, for example in the 20 MHz to 50 GHz range comprises a MESFET distributed amplifier (22) having a gate and a drain transmission line, a first hybrid circuit (18) to apply a first and second input signal to opposite ends of the gate transmission line, and a second hybrid circuit (26) connected to opposite ends of the drain transmission line to receive and combine first and second output signals from the drain transmission line to provide an amplified output signal. The use of two input signals travelling in opposite directions along the gate transmission line increases the gain which can be achieved in the distributed amplifier and reduces the noise component of the output signal. It is particularly useful for enhancing the performance of a distributed amplifier containing only a few MESFETs.