The amplifier and discriminator described in the paper are component parts of a completely transistorized frequency-lock loop. Each is contained in a compact potted block; the amplifier uses printed circuits.Interesting features of the amplifier include its excellent a.g.c. performance and its elegant circuit configuration. The input stage uses biased silicon diodes as variable gain/loss elements; a gain range of over 90 dB is obtained, with negligible distortion or change in overall response, and with a substantially linear control characteristic. Following this are three synchronously-tuned common-base stages, with finally an output stage and detector. The maximum overall voltage gain is 106, with a bandwidth of 4 kc/s. The complete amplifier contains 11 transistors and 8 diodes.The discriminator is unusual in combining crystal control with a linear response over a wide bandwidth. Basically, discrimination is effected by beating the incoming signal with a reference signal generated by a local crystal oscillator, and measuring the beat frequency. The system is very suitable for use with transistors, since most of the circuits handle only low frequencies. The complete circuit contains nine transistors; a linear response of high slope and excellent zero stability is obtained.