Although in minor keys we usually use the notes from the harmonic minor scale to build the dominant chord, it’s also possible to build a different kind of dominant chord, using the notes from the melodic minor scale.
In the descending melodic minor scale, the 7th of the scale is not raised by a semitone. Using this note creates a minor dominant chord, instead of the more typical major dominant.
The minor dominant triad in A minor contains the notes E, G natural and B.
Because the chord is minor, we use a lower-case v to describe the chord.
The minor dominant chord is rare compared the major version. It is sometimes used to harmonise the top half of the melodic minor scale (the section of scale can appear in either the melody line or bass line), or it is sometimes used as a pivot chord when modulating to the relative major key.
Take care never to use minor chord v at the point of modulation (V-I), or at a cadence (V-I, I-V etc.)
In Bach’s chorale no.16, in B minor, the B melodic minor descending scale provides the bass line through bars 1 and 2. Minor chord v (F# minor) is used to harmonise the scale on beats 2 and 4 in bar 1. Notice however, that major chord V is used at the beginning of the piece, and at the cadence.